Thanks for the reply David. Unfortunately its too late for the testing part as the gearbox has already gone pop and I cant select any gears at all! Luckily it appears to be stuck halfway between 2nd and neutral so it still rolls....
My question was based around the fact that the current gearbox is a TY752VN6AA, but is in a v2 STi which should have had a TY752VB4AA box in. I had assumed that everything had already been converted to take the 6AA but then I saw your post about the diff going and it not really being noticeable and started to worry as I dont know the full history of my car.