Check emissions

Hi, I have a Modus 1.2 (54 plate). I also started to get a check emissions warning light (computer said misfire in cylindar 3). Had the coil pack replaced.... didn't fix the problem so took it to main deal.... they replaced an injector and adjusted the valve clearance. 8 months later same problem. After the dealership had it for 2days they reckoned it was a valve problem but they would need to strip the head to check.... total to check and replace valves £1900 (on top of the £400 i'd already paid of the injector and clearnace and £50 for the coil). Finally took it to local garage who replaced all exhaust valves for £1100 as one value was burn't out.... its strange that renault changed the cylinder head design of the engine from 56..... Neil