After getting a P0353p reading on my DTC reader and after some advice from the 260 forum I needed to investigate the COP (coil over plug) on no3 cylinder. Typical as that seems to be the most hidden of all plugs as it is well covered by the large air duct from the air filter to the throttle valve. Fortunately I only needed to loosen the circlip at the top end which was enough to move the whole duct over enough to get access to the plug. Unscrewsed the COP fixing bolt after releasing the wiring plugs and pulled out the COP. I immediately noticed a hard black compound on top of the plug to the casing which I cleared.
Swopped the COP with no5 cylinder and refitted all the parts. Started the engine and it still sounded the same but I won’t know if the misfire has now moved to no5 till i use it a bit and get a new DTC reading.
Have got a set of plugs on order and a new COP ready.