Hi Kev - I'm also in Chester (just replaced a chain on a Corsa but not done a 2.2 as yet).
As will be clear from many people here Benny is first class at this and is based in Wolverhampton - which doesn't take long down the A41. Depends if you feel happy to drive it as it is.
Alternatively if you go on the VVOC website there's a guy on there called Appy who is mobile mechanic based in Cheshire. He has done quite a few of these. Not always easy to get hold of since he's a firefighter specialising in releasing people from crashes - so he can disappear without warning.
Based on my experience I'd only fir the genuine GM kit.
PS If you want to listen to it through a stethoscope let me know. Might track it down a bit.