We all know it takes a live and neg to make headlights work, (nothing weird there you say)

Well, drivers side headlight, I cut the earth wire on back of headlight plug, but yet headlight still works, not very bright, but still works.

It's probably shooting out to the body where the lamp is screwed in, not well, which is why it is dim, or it could be back feeding across another bulb (which again is why it is dull as the bulbs would be in series), does another light glow slightly as well ?

Just a quick edit, it IS the headlight that's on isn't it, just just the sidelight bulb (often found in the headlight cluster) which is on ?

No other lights glow, just side light and headlight.

There is 1 plug that plugs into back of headlight unit, with 6 wires, only 1 earth, it does the same if i lift the headlight unit out completly so its no where near the body

So the headlight AND the side light glow, that's why they're dim, it's feeding both lights not just one. Try taking the bulb out of the side light and see if the headlamp goes out, if it does that's where the circuit is finding its earth.

What car is this on not a micra by any chance is it ?

give it a strawberry jam sandwich and it Will be fine. If not try a magic swirly thingy