Indicator Nut

The nut holding my rear left indicator has vibrated off leaving the indicator hanging by the wire and rubbing against the rear tyre.

Gonna gaffer tape it for now but need to source a replacement nut.

I went through a box of nuts we have at work and found one with 8mm outside diameter but the inside diameter was too small.

I remember doing it up with an 8mm spanner so am fairly certain about the outside diameter but no idea about the internal.

Anyone gotta clue or am gonna have to measure up?

Also where can I get one this evening in London? Wicks ?

Had the same problem a while back. I stopped off at a KTM dealer and they went into their box of nuts for me to try out. Replaced both nuts with nylock ones. Hardware stores didn't have the right sizes so it must be a motoring nut thing.

8mm spanner may be too small? Check against the other one with a spanner.

If I recall, it's about a 10mm thread, which means a 12 or 13mm spanner.
Spanners are always bigger than the threads. Measured across the 'flat' of a bolt.