Well I decided to cut off the worst of the kink on one end of the spring (about 4 coils) and eventually got it back together! I had already sleeved the spring so left that in for now, and have a working hw77 again until the tin bum kit arrives - phew!
I have to say it fires lovely and smoothly now, but am pretty sure this is because I have reduced power considerably rather than my gun-smithing skills!
Just glad it's back together and working again, as my almost 5 year old was distraught when he saw it in pieces earlier.
Cheers, Matt