Bad earth maybe?

Having a slight electrical issue.

A but confused here. my left hand tail light works on its own fine, and the brake light filament part of the bulb works fine on its own, but if i have the headlights on and i press the brake pedal, both filaments go out. The right hand side works just fine. Now I have had the bulb out and got the multimeter on the holder and i get 13.5V from both connections in the bulb holder when both headlights and brake pedal are pressed at the same time but when I put a bulb in, I get nothing. and bulbs are fine as iv'e tried 2 new bulbs.

Bad earth maybe? dodgy bulb holder?

That's usually a symptom of an earthing problem.

Thats what i thought, any idea on the earth location for the rear left hand lights?

under the rear panels in your boot

Classic earth problem, measuring with the meter a good start but with bulbs removed theres no load.
usually local earth screw or oxidized bullet connectors, worth cleaning but sometimes plugging and unplugging them a few times can work