Which LET? Z-LET or C-LET?

Can't see an F20 fitting in a Mk5 personally... you changed pedal box to cable operated clutch?

It its a Z-LET then it should be on either an F23 or a M32 - in which case shafts from an Astra VXR, 1.9 CDTi etc will fit.

If you have actually stuck a C20LET with F20 in a Mk5 Astra, you need your head looking at! Although 52 plate could be Mk4 I guess... in which case it should be an F23?