Sorry for the radio silence.
The only offer was £5681 and I keep the car. I tried haggling but they did not budge.
The car is obviously still driveable and I plunged out some of the bigger dents so it looks slightly less @*!#.
I did buy the panel that GranTurismoEra linked to but when I went to get a quote the chap pointed out that the door was a bit damaged internally at the bottom and would probably not look very good if I went down that route. Sent it back and got a refund.
So, I'm still looking for a replacement door, the colour is twilight grey and I guess not as many of those were sold as I haven't seen any on the usual places. Some breakers on eBay seem to have cars that might fit the bill but the photos aren't great and they don't always specify the colour. Plenty of silver doors, so I'll probably end up with one of those.
I got a quote for - fixing the rear quarter panel, changing the doors (incuding the workings, lock etc) and respray for £900. Seems okay to me but I'll get a few more quotes. Assuming the door will be about £200, I'm looking at £1100 to get back to where I was. That gives me £4581 cash to cover the loss of value as it is a CAT N.
I've done some soul searching and realised that I bloody love this car, so I'm keeping it.