Or the exhaust manifold!
when leaking it draws fresh air in and the hot exhaust gasses detonate it.
Or excess carbon build up in the combustion chamber.
There's quite a few possible causes.
I'd start with a smoke test of the induction system.
problem now solved. according to the service history ( so called)quick fit auto services did the servicing on it for the past three years. during that time they had done work on intake manifold and exhaust front pipe ,so that rang alarm bells. removed intake manifold .NO GASKETS!!!! just sealant. two 10 mm nuts missing so sorted that mess out and then went to exhaust front pipe / manifold. two studs snapped . so thoroughly enjoyed sorting that out .runs fine now. many thanks for the very sound advice,,,,,ron.