Ok, so I have a highmounted stoplight in a 76 clubman. I've noticed the LED brake light comes on fine with the parkers and headlights off, however it doesn't work with any of the lights on. This is less than ideal, unless I restrict myself to daytime use only, or make sure no one is behind me at night (its only a 998 so prob not going to happen). Anyway, the normal globe brake lights still work fine with parkers/headlights on or off and electrics are otherwise fine (as far as I can tell). No other mods to the car.

Thoughts, experiences and any help most appreciated.

check the earth for the high mount stoplight. Chances are it's earthing through the stop light circuit, so works when the lights are off, but won't work when the lights are on

Cheers for that! You were bang on the money. I traced the wiring down and ended up having to remove the tank to fix the issue. It was earthing through the parking light circuit just behind the LHS tail-lamp assembly. Also fixed the breather hose that was getting squeezed shut by the tank strap while I was there

no worries... it's very common on modern cars too as their wiring and connection deteriorates. I often see cars driving at night time with one tail light going on and off as they brake - same problem