While conducting some additional research on the Tachwüll (what can I say, I read all I can on Patagonia and sometimes come across interesting information), I came across an article by Mateo Martinic : "Documentos inéditos para la historia de Magallanes 'Memorándum Referido a los Patagones'", published in 2007.
Martinic presents it as an "unpublished" document which was sent to him by Mrs. Jean Cameron, in charge of the Archives at Falkland Islands, which bears the title "Memorandum respecting the Patagonians". The notes did not indicate the author or the year it was written, and Martinic correctly (as we will see below) dates it to around 1840-41.
There are two parts to this story, one regarding the "unpublished" letter, the other about the "Tachwüll". This post will deal with both.
The "unpublished" letter
Martinic was wrong, the letter had been published before, in 1843 by the British Parliament's House of Commons.  It was written in 1842 (Martinic was quite close in his estimated time frame):
The above notes were furnished to the Lieut governor by Captain Allen F Gardiner RN who collected them in Patagonia in the early part of the present year 1842. 
The text was mentioned again in print in 1964 . So, it was not "unpublished" after all.
The positive thing is that it mentions some people named Tatchwell, and states that they are natives, not dwarves.
Tatchwell or Tachwüll
I will quote Gardiner's text in full (the part that mentions these natives). By the way, Allen Francis Gardiner (1794–1851) was a British Royal Navy officer and missionary and he starved to death in Tierra del Fuego, after an unsucessful attempt to set up a mission among the Yaghans.
First he stated that "There are five tribes of Patagonians [...] and one on the sea coast west of the Cordilleras [...] [these are the] Tatchwell Cho karro west of the Cordilleras 4,000 [population] "
I am at a loss to explain what Cho karro is. But lets go to Gardiners memorandum; he then gives an account that he got from a man, his wife and son, who belonged to the "Tatchwell tribe":
My comments are in brackets:
The district by the Tatchwell is wet and rainy and heavily timbered with trees of great size tents dress and stature is similar to that of the other Patagonian tribes they have canoes but these are only employed for crossing rivers and are merely a light covered with guannco skins.
[Note that they are "the same size" as the other Tehuelche, these Tatchwell are not dwarfs]
They use no paddles but are towed across by their swimming before with a lasso attached to their tails. This country is represented as about north west of Oazy Harbour [Which is located on the Strait of Magellan at 52° 30' S; 70° 31' W ] but in order to reach it is necessary to travel from thence considerably to the northward as the pass through Cordilleras in that part is better suited for horses than one farther south which they do frequent.
[This is correct, there are no passes south of Lake Pueyrredón 47° 16'S. Because of the Southern Continental Ice Field that reaches from the Pacific Ocean coast to the Andean Peaks between lakes San Martín and Mount Payne.]
In the neighbourhood of this pass there is a large lake with an island in it. This of the country and around the lake is inhabited by a tribe called Thit titch whose chief named Tchucato.
[There are several lakes with Islands in them in the area: Lake Belgrano (47°51'S, 72°10'W), San Martín (48°47'S, 72°50'W), Pueyrredón (47°14'S, 72°05'W) and Buenos Aires (46°24'S, 71°45'W. Though Martinic believes that the natives were talking about Lake Nahuel Huapi (41°01S, 71°27'W), which is too far north in my opinion.
I have heard the name Thucato before, but not Thit Titch]
They are more numerous than all the three Choanik [Tehuelche] tribes wear ostrich feathers on heir heads woollen ponchos and a sort of trowsers cultivate ground and have numerous flocks and herds of sheep cattle and horses. Their is different from that of the Choanik they trade occasionally with the Spanish settlements. Through this people they pass on their way across the Cordilleras and direct their course towards the south west [to go to the Strait of Magellan].
[These Thit Titch natives are without doubt, some Mapuche group, because these were Farmers, cattle breeders and weavers. The Tatchwell live to the west of the Andes]
The lake above mentioned they call Chobit it is about seven days on horseback from Oazy Harbour and thence to the Tatchwell on account of the ruggedness of the route and the forests would occupy ten days more. Another tribe called Eaks was also mentioned by the same individual they a district north of the Tatchwell between the Cordilleras and the sea [to the East, the Pacific Ocean as we will see below] but there is intercourse between the two people as their language differs and in one direction a considerable river which they cannot cross in their canoes runs between them.
[Seven days at about 35 km a day (roughly 20 mi.) is 245 km or 150 mi. Then another 10 days or 350 km (217 mi.), the lake's native name is not recorded though, Chobit sounds a lot like Chubut, the name of a Patagonian River and Province.
The distance from Oazy to del Toro Lake by Mount Paine is about 240 km. And this lake has an island in it. An additional 350 km places the territory of the Tatchwell in the area between lakes San Martín, Belgrano and the mouth of Baker River on the South Pacific Ocean.]
The Eaks are a shorter race than the Patagonians and are habited in ponchos. It appears to me probable that the lake called Chobit by these people is the same from Viedmas testimony is marked on the maps Capar [current lake Argentino or perhaps lake Viedma] and the wearing of feathers in the hair and other circumstances related by the Tatchwell man Wao principally the former is I think sufficient to identify the Thet titch with the Pewenches of which nation they are probably a tribe. 
Intrigued by this text and the names of different native groups (new to me) I did some further research: 
The text above was written in 1851 by the South American Missionary Society (of which Gardiner was its first secretary in 1844). Below is the "text version":
the Tatchwell are found far to the westward near to the eastern slopes of the Cordillera fronting the archipelago of Madre del Dios as nearly as I could ascertain from the account given me by a native of that part of the country whom I met in 1842 during my stay in Coazy [sic] harbour . 
Which places these Tatchwell on the Chilean side of the Andes, close to Madre de Dios islands. These are located south of Taitao, and of the Gulf of Penas, (50° 15' 47 S, 75°18' 30 W). This is between del Toro Lake and Baker river's Mouth. To the west of the Continental Ice Sheet. If so, the Tatchwell were living in a dreadful environment, and were very likely Alakaluf canoe people which were not Tehuelche but a differente group, with their own language and culture.
Regarding the Eaks which lived to the north of the Tatchwell, the name does not appear in any book or paper regarding Patagonian natives however, I did find an interesting remark  which gives the word used by the Northern Tehuelche (Günnuna Kenna or Gennakenk ), also known as Pampa (or Puelche, the name given to them by the Mapuche): 
The Pampa called these groups with the following names: Tehuelche = Ehnakena; Mapuche = Telunakena or Iaskas.
So here we have a word Iaskas (the last "s" is the Spanish plural), whose english pronounciation is easkah, which sounds very similar to Gardiner's Eaks. Furthermore, the fact that they wore woven clothes and had domestic animals and were farmers is a clear indication that they were Chilean Mapuche (the only sedentary natives in Patagonia). However Martinic in his paper  says that these may be "Pampas Indians" due to the different language (maybe he is referring to the Mapuche speaking "Puelche" and not the "Tehuelche" speaking Gennakenk which bear the same name.
There was a native group, of Tehuelche stock living to the west of the Andes somewhere between del Toro Lake and the mouth of Baker River, in Chile. They lived south of the Mapuche (Eaks). Were similar in dress, tents and size to the other Tehuelche of the Patagonia. They used light rafts to cross rivers. They were not dwarves.
The problem is that the only known natives in that area were not Tehuelche but Alakaluf, another totally distinct group, with their own language and clothing and way of life. They did have canoes but did not venture far inland. They did not have horses and did not ride from the Strait of Magellan to the Madre de Dios Islands.
Regadding my dwarf, the Tachwüll, these mysterious natives, the Tatchwell may have recieved their name from the dwarf, as they lived in the same area, in the mountains and forests which the Tehuelche feared and seldom entered.
This deserves some further research and a letter to the journal (Magallania) to rectify Martinic's paper.
New. Nov. 2, 2010 Today I wrote to Magallania, sending them a "comment" about the author and publication in 1843 of this paper as well as some interesting information regarding the "tatchwell". The letter is online, (in Spanish) at the following link:
Comentario sobre un Memorándum inédito referido a los Patagones (Martinic, 2007) by Austin V. Whittall.
 Mateo Martinic B. DOCUMENTOS INÉDITOS PARA LA HISTORIA DE MAGALLANES "MEMORÁNDUM REFERIDO A LOS PATAGONES". Magallania [online]. 2007, vol.35, n.2 [citado 2010-10-27], pp. 159-164 . Disponible en:
 Accounts and Papers. COLONIES. Session 2 February 24 August 1843. 33. VOL XXXIII. House of Commons papers, Great Britain. Parliament. House of Commons. HMSO, 1843
 Boletín de la Academia Nacional de la Historia, (1964), Academia Nacional de la Historia (Argentina). vol. 35, pp. 280.
 South American missionary society, (1851). Remarks on the Aborigines of South America from Personal Observation THE PATAGONIANS. In The Voice of pity for South America . London, J. Nisbet & Co. pp 92+
 Bórmida, Marcelo, and Casamiquela, Rodolfo (1958). Etnografía gününa-kena. Testimonio del último de los tehuelches septentrionales. Runa, Buenos Aires, vol. 9, nºs 1-2, pp. 153-193
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©