[NigelK]

All 4 cylinders seem to be burning the same. Car almost died at the round about again today. As far as I am aware the only thing that could cause pre det and power loss during a head gasket change is if the timing was off.

Would a reason for pre-det, pinking, be that it is running too weak a mixture? If so, why following a head gasket replacement would it be running lean? But you saying the plug colours look OK?
Worth doing a compression test?
The 220bhp - that is a dyno figure presumably? Worth doing another dyno run but with an AFR read at the same time? See what the AFRs are doing? And what bhp you are actually getting?