I've heard good things about Colin Ferns (
Mercedes in Richmond - Used Mercedes - Service - Command SatNav Retrofits ) who are in Richmond
If your car is only just out of warranty you should consider dealer servicing as it keeps your rust warranty valid and also they are much more likely to do goodwill repairs on MB full service history cars. (Paid 90% of my suspension claim in a 4.5 year old SL)
Cheers
Richard