@Graham *******
did the gf or owner report your stay with her at immigration
If not there could be an issue at immigration if you try to do yourself.
Last year a mate of mine stayed at his friends condo, neither reported address of stay. When he went to get 30 day extension by himself he was fined 2000b for non reporting.
Using an agent maybe best option.
Maneerat soi
****
So good visa
There are plenty that can help