The rocker cover gasket is an easy DIY job. The part costs about £8 per side from memory. Its worth doing both at the same time, and also replacing the cam oiler tube fittings while its accessable. They are about £3 a side.

If the car hasn't recently had the timing chain and guides changed in the last few years, then thats another bargaining point. The guides are made from plastic, and with age and heat they go brittle and can break. If they break you might get lucky and nothing will happen, but read some of the horror stories on this forum. Its like the cam belt breaking on a belt driven cam car.

I'm afraid I don't know of any London mechanics who specialise in MB. There is a guy in Kent who appears to know his stuff though, his name is Chas - He runs a garage called 'SEC-Shop'.

Cheers,
Ben.