Help I've done something really stupid!

[bigalw35]

I was hoping it might be a copper alloy with some magnetic residual, just an idea.

If I was the OP I think I would not spend too long on trying to fish it out but go for a replacement. With old wires and insulation you never know what condition they are in.

Sorry but alloy is not magnetic either, your better off with the grappler

 

[Fdhadi]

If it’s a top marker light you could replace either side with a white reflector. Obviously tape the wires on the side where you have 2 wires and then fix these to the reflector so they don’t drop down.
Do the same with the other side where you have lost the one wire.

It will pass an mot with reflectors in place.

 

[gus-lopez]

This may help Grabber Tool from Amazon

I thought everyone had one of those indoors?
P.s. I get 10 here for that price

Dane - thanks for the link to the grabber tool. First I have to locate the lost wire using the inspection camera and then, assuming I can find it, I will need something to pull it back. The grabber tool looks like the way to do this.
By the way, I've found an inspection camera that I can borrow tomorrow so all it not lost... just yet.

No you can just poke around with the spring loaded grabs open and keep shutting them until you grab something.