Just to bring things up to date.....following Dieselman's suggestion I bought 2 litres of Redex (for diesel) and poured it all into the tank when it was down to reserve, and ran the engine for 1/2 an hour. The fault code was then erased using an OBD2 scan tool and when the engine was restarted, the "check engine light" went out

BUT it came on again soon after

so obviously the fault is still there, and I suppose that the next step is to remove cylinder 3 injector and replace it with a new one (cheaper from the Bosch Service Centre, perhaps?). Incidentally, we've had the car from new, and this is the first trouble it has given.
Does anyone have any experience of doing this job on the current type of A class diesel engine (W169 A160 CDI). There is quite a lot of info on the site about working on C class engines, and maybe that is relevant to the A class engines also, but although I have lots of experience with petrol engines, I have none with diesels.
Any pointers would be most gratefully received, plus whether it is a reasonably straightforward job, and whether any unusual tools are needed to get the injector out and its replacement in.