We had the same fault recently and it was a bad connection on the earth terminal on the back of the bulb.

 

as above - it will be a bad earth

check the connector that attaches to the bulb - it might have melted/rusted and need replacing

if you do replace it, make sure it gets wrapped up well with something waterproof to stop water getting into it

 

Yeah just fixed this problem yesterday. Was a bad earth, if you pull the connector off of the bulb you'll see the plastic around the black wires connector will be all melted. I just made my own ground wire for the bulb and connected it to a bolt in the engine bay.

 

Thanks for all the quick replies, it was the connector block/bad earth issue...managed to get it sorted by wrapping the spade on the bulb in tin foil to get a better contact...bodging I know but had to find a quick fix as its in for the mot on Monday and will hopefully do the trick until I can get a new connector block

Cheers

Stuart