Thanks for your feedback everyone - it seems that a diesel *can* be okay, but the weight of opinion says, more often than not, it won't be ideal and that a petrol engine is a better bet for my use case. I guess that, in my heart of hearts, I knew that was probably the case, but didn't want to dismiss a vehicle I was smitten with.
Forgot to mention that, prior to the test drive, the first thing I asked the salesman was whether the vehicle was a petrol or a diesel (the windscreen info just said "hybrid"). He was quite dismissive and almost snorted that "all hybrids are petrol mate". It wasn't until I got home and did some research that I discovered he was wrong. If he'd have answered accurately (honestly?) I wouldn't even have bothered with the test drive and I wouldn't have been in a position of having to walk away from the car.
Anyway, I got a follow-up call from the garage this morning and I gave them short shrift.