Ford Focus Cc3 Engine System Fault

About a month ago my CC3 started juddering and came up with system engine fault. Look it to local garage who said two spark plugs had been fitted which were the wrong size. Replaced them and all seemed OK. About 2 weeks later happened again, garage closed so took it to another who said the error stated throttle body fault. They cleaned that and all OK. Next day happened again. They sent it to a specialist with the Ford software who said two coil packs needed replacing plus an engine service. All looked OK but after a few hours error again. It was sent to an ECU specialist who after 2 weeks of diagnosing the fault and making me buy a spare key for £210 (!) said it was the throttle body - guessing I didn't need that key after all. That was replaced today with a brand new one from Fords and same problem. All 4 coils replaced today, same problem.

Garage are now going to check the ECU has a signal and if it does think it’s a mechanical fault. They said cylinder 3 is slightly down on compression and the car is using a significant amount of oil (40 miles since service and it’s just above warning level). They are planning to take the top off the engine and go from there…

Been without the car for nearly 3 weeks now and am over £1000 down without the problem being fixed – any suggestions???? I am guessing it needs to go to the Ford dealership now but am hoping the garage I've spent loads of money with already can fix it or I'm back to square one with the grand down the drain.