Funnily enough I solved this yesterday. The issue was indeed an earth issue but actually due to the fact the earth route is through the body of the bulb and the metal retaining clips for the bulbs were just about knackered
I managed to make repair using some springs and wire until I can find some replacement clips. Looks like a trip to the scrapyard is required. Also it was much easier to sort out once I completely removed the headlights. This in itself was a bit of a faff as to make access easier I had to undo the bonnet retaining thingys.