Once again thanks for all the comments.
The mirror was locked up, but this was not the root problem. The aperture lever had been damaged. It was thought this might be due to a missing stop screw on the lens, but all lenses are OK, so not sure why this has happened.
The service from Nikon has been fantastic. I took advantage of the ability to return at no charge via Parcelforce and sent it on Wednesday. Thursday, I received an email to say it had arrived. I phoned about 9.30am Friday as I am due to go away in a week, and the estimated time for return was 2 weeks. They said they would try to speed this up. Later on Friday morning, I had a phone call back from Nikon to confirm the problem. Unfortunately it was not covered by the Warranty but they gave me 25% discount. A further couple of phone calls and photos to try to sort out if missing stop screws were the problem followed, and then at 4pm I received a further email to say my camera was repaired and had been dispatched, so am expecting to get it back tomorrow.
Exceptional service.