The story I got from the people at Dr Entropy's link is that they make all their looms from original ones and do not deal in diagrams. Seems odd to me but not for me to judge. They are quite successful what ever way they do it. I got the same story from the UK wiring suppliers.
Wiring Looms
That said, Lotus harnesses are difficult or at least seem that way to me. The harness on my Plus 2 does not match the diagram in the manual (my VIN falls into a gap between models) nor the harness I bought (same issue probably). In addition the cars got quite complicated with AC, power windows, and other "improvements". I've been building a new harness, from what's left of the old.
The British codes above will help with trouble shooting basic circuits but won't hep much with the electrical "extras". And in my experience Lotus was not too fussy about using the British codes.
Best of luck,
Tom