I am self-employed. What evidence would the UK embassy specifically require from me when assisting my thai girlfriend in getting a fiancee visa to come and live in the UK?

Your last 2 years' accounts. Failing that a letter from an accountant which states how much you've earned over the last 2 years.

Scouse.

If you're a contractor, you can get by with a letter from your agency giving your rate, when your contract started, when it runs out, and likelihood of renewal. (at least - that's what I did...)

Of course, if you have a copy of the accounts anyway, take them along. There's no guarantee they'll take the agency letter...