Swap the bulbs over (along with the ignitor box on the back of the bulb) from the working headlight to the problem headlight, if that doesn't fix the problem headlight then the bulb is fine its the ballast underneath the headlight that's the problem.
Basically just swap bits between them until you find the issue.
Cheers for the help, changed the bulbs and packs over, where do I get a ballast from? as Im assuming its that as I swapped them over it worked.