With big thanks to Racer_S ... :D ... 8)
The tool is DirectX 9 only at the moment ...
Download the tool from here ...
CLICK ME
Unzip the tool anywhere you like ... I'm using XP and mine is in the games main directory/folder ... C:Program FilesUbisoftAssassin's Creed ...
Run the tool ... leave it running ...
Set it as follows for TH 3840x1024 ... or your desired TH resolution ...
Choosing your version of the game ... mine is the UK shop bought version.
Just input the res you are using for lower TH resolutions ... the other settings should stay the same.
Start the game ...
Press the * key on the number pad to enable the tool ...
If for any reason the graphics get messed up going from menu to in-game or back ... just disable the tool with the / key on the number pad and re enable it with the * key ...
For those running 5040x1050 you need to set it like this ...
Aspect: 0.2056 (same for any 3 x 16:10 res)
Left - 0
Top - 0
Right - 5040
Bottom - 1050
That's it ... enjoy ... :D
The HUD is stretched but it can be seen a bit better than it was without without the tool (because it was annoyingly behind the gaps between the screens).
It can also be turned off in the games options if you so desire.
Edit : Added the 5040x1050 info