I think I have it sussed, then of course something happens to prove I know nothing.
The back bet (£25) was not fully placed, but then the lay bet comes in, treating it as though the full back bet had gone in, I know it's something I haven't done, but what haven't I done?
I'm pulling my hair out.
Quite simple really, make sure "Place fill or kill bets" is ticked and put it at like 600 seconds delay (you can put the delay as long as you like) under Parameters in your "Back Selection 1" rule.
Once that is done, the lay bet will only fire once the back bet has been filled.
Once that is done, the lay bet will only fire once the back bet has been filled.