Having checked it's the T15 bit that fits so I guess it must be a M3 then??
Hmm, that's strange. Standard torx size for M3 is T10, T15 is for M3.5 so I suspect you have a non-standard M4 or M5, M3 or 3.5 seems far too small for a wing bolt.
Best way to check is to put a spanner on the nut underneath, this will tell you. if you need an 8mm spanner it's M5, 7mm for M4, 6mm for M3.5
