Swap the two ballasts over to see if the problem swaps side. If it doesn't do the same with the bulb. If it still works intermittently then check all the connections are secure and clean as all it could be then is a bad/dirty contact.

As 'Jack' said, swap the ballasts and the light units around to determine where the problem lies.

Dr Bearface. Where did he say the HID's were aftermarket? HID's have been fitted standard to many vehicles since 06/07.

Hi so as they have been fitted to a car that is too old to have the lights covered by a warranty. they are a illegal and be if it has to have an MOT it will fail as they should not be fitted.

swap sides with the bulb

One light failed to turn on sometimes, it got worse and worse so I eventually replaced the ballace and it seemed to work correctly. Its been about two days and now it is starting to do it again but not as often as it did before? Could it be a bulb?