So, I called Mercedes yesterday and spoke to their main service center (not my local dealer).I suggest that you walk into the dealership with a photo ID and your V5C, and ask for a printed copy of the DSB. This will provide you with a starting point, and also from that point on, you'll always know what's due next without having to rely blindly on dealers' quotes.
They told me it had a 'B' service when I bought it in Feb/Mar, and so it needs an 'A' service. So the £650 was for an 'A' service....but....it's mileage also means it needs an air filter and fuel filter change too - so the price includes that too.
I'll probably go with the main dealer this time and get the 12v battery warning sorted at the same time (so probably looking at a bill over £1000) then maybe look at an independent next time.
Thanks everyone for your help.....