Thanks for the quick response Bobbin-man,

You could be onto something, our car does tend to get filled up on the shopping run, so it might be worth switching.
It's into the dealer next week, so if they find anything I'll write back.

The service guy did agree with me over the phone that it would not be "normal" behaviour, but whether they can find a fault is a different story....

I agree, the car does seem to be really loosening up with more miles, this is my first diesel and I never really noticed it in previous petrols.