Plot thickens!! Had car in today at main dealer who has stated that the problem can be 2 things. What was originally a plug question has now developed into Injector/Valve problem. Fault message "Injection System Faulty" and ESP faulty(why? they can't be related surely)
Tech says solution (maybe??) change injector rail (£1100 !!!)
or if that doesn't fix take head off and check valves (£700 labour plus bits). Still can't see why the car goes like stink gives 30 mpg and accelerates as if the engine is fine. Perhaps I should move this to another thread or start a new one. Anyone out there who can shed some light?