Spoken to my dealer this morning, who has already had my car up on the ramp for a couple of hours. The problem appears to be a faulty injector.
Apparently - Modern diesels are a lot quieter, as they don't let all the fuel in at once - which led to the infamous knocking of a diesel engine. A new generation effectively drip feeds the fuel in, and greatly reduces the knocking sound. The suspicion is that my faulty injector is letting the fuel go in all at once, hence knocking like an old diesel.
A replacement injector has been ordered, and is due to arrive and be fitted tomorrow. If all goes smoothly, I may have my car back tomorrow
Keep them fingers crossed!