Thanks for the response, I appreciate your input. I haven't noticed any significant power loss, but that being said I haven't really driven the car properly since the problem fully manifested, just a couple of short local trips at >50kph.
I hadn't considered clutch as a possibility, but nothing else really indicates that it might be as far as I can tell. Unfortunately I've only had the car around 6 months and service history prior to that is patchy at best so I've no real idea if the cambelt has been done on schedule or not, but I know there's a chance a failing cambelt could be throwing out the timing. However, like I said, my knowledge of diesels is limited, but the high pressure fuel pump performance is clearly all over the place, which is what leads me to suspect the issue is fuel system related. The fact that the issue came on quite swiftly led me to think it might just be a clogged injector, but I'm pretty sure that wouldn't be leading to over fuelling, which seems to be the case in cylinder 4.
I've got it booked in at the local garage on Tuesday. They've got a good rep, so hopefully they'll be able to figure it out pretty quickly, but I'm still open to suggestions from folk here who can offer some insight.
Thanks again.