During my frustrating trip back from Leicester, I noticed that when I braked, and also sometimes when indicating, the rear fog light warning light came on.
I know some cars lights behave strangely when a light is out due to common wiring but I checked all rear lights and everything seems fine...Any ideas before I get my multimeter out in a pointless search?

The fog light should be wired so that it only comes on when position lights are on, so I suspect there is a short wherever those 2 are linked together.
Or, do you have a tow bar and trailer wiring?

usually an earth problem

No tow bar, headlights weren't on so suspect dodgy earth at rear maybe....