Thank you for you kind words and advice Cilla and Angela. I will certainly bear the phantom itch in mind if I have to go back the vets.
Gizmo is doing everything he used to before he lost his leg. As we are Semi - rural we have lots of cats around and he had to fight for his 'land' back as soon as he was given the all clear to go out(under the watchful eyes of mum and dad of course).
It may have been coincidence when the spots appeared, I have put it down to that anyway. The constant grooming of the healed wound area is a mystery to me, maybe it is phantom pain/itch and he is trying to make it better.
For now though he seems more settled probably due to the steroid and antibiotic injection this week.
Again thanks for the advice and will keep you posted x