I've got a 218 diesel (without the turbo) and the damn thing is a pain to start when the engine is cold. I've changed the glow plugs, the electrical rail that goes accross the 4 plugs and its still the same. Its tries to start but when it starts it runs erratically and sounding like its running on 3 cylinders. I've done a compression test and thats ok any idea's !!!!!!!!!!!

Have you checked the glow plugs are actually recieving power? Sometimes the glowplug relay packs up (particularly on the later L series cars).

Also try pushing the priming bulb a few times before you attempt to start the car. If this cures the problem then it is an airleak problem rather than a glow plug one.