Thanks for such a quick suggestion - sorry for my delay. I took the car to my local independant shop and they hooked up the diagnostic tool and it turns out that the O2 sensor was faulty. This was replaced and at the moment everything seems to be happy - problem solved. There also seems to be a slight increase in power from the engine - maybe my wishful thinking!
On another unrelated point, I would like to thank everyone who gives up their time and shares their knowledge in this forum - it is a great help to someone like me who has mechanical knowledge but is new to Mercedes. The other evening I read for about 2 hours all the threads relating to central locking vacuum leaks and the next day managed to trace the fault on my central locking down to a leaky fuel flap actuator in about an hour - the new part is on order.