What about a plain nut and a couple of washers or even a wing nut if you can find one or try a scrappy. You could try lashing it into place temporarily with a couple of nylon ties or such like.

Have yoiu tried looking inside the lamp aperature it may have fallen inside - a torch and a small mirror may be helpful.

 

Many thanks madnoel10. I hadn't thought of looking in the lamp aperture. I've had various thoughts about glueing nuts together to make one deep enough but I confess I've not taken a look at the one on the other side yet to make a judgement. I'm guessing that scrappies want to sell the nut complete with the light fitting.

 

scrappys wont mind if you just take the nut aslong as you dont ask them,if there is a problem,tell them dave said you could have it.

 

Thanks davdav. I was intending to visit a scrappy to see if they could supply this particular nut. Instead, as I couldn't find the old one anywhere in the car, I did as suggested and used some washers and an ordinary nut of the appropriate size.

Thanks to everyone who took the trouble to respond to my query. Much appreciated.