Problem found and being fixed as we speak blokes at horner bros. Have been excellent really friendly bunch.
The problem, and you'll like this one... Was not the hid ballast, was not the bulbs, was not the horn.
It was an earthing and corrosion issue caused by... My k&n typhoon metal airbox. where the wires skirt round the airbox, they had actually been pushed under one of the edges of the airbox. And as it was such a tight fit, it had actually started to wear through the wodge of cables, and then the air box had worked as bridge between the lighting circuit and the horn circuit.
So the blokes have cut a small section out of the airbox to allow for the wires. Have repainted the cut onthe box, re insulated all the wires as he damage was only minimal.
I now have working headlights, working horn, and no more issues.
One othe thing they picked up on was my grounding kit where it connects to the negative terminal, they didn't rate the gold connectors as the Allen key connectings work loose. Which is exactly what mine was also doing. Hadn't caused problems yet, but they explained that it would, so they are also soldering in a new and better connector for my grounding kit to help avoid any other problems later down the line.
Total time taken to fix just under 2 hours hopefully, so I'm guessing a final cost of about £90. Don't know exactly yet as they are still working. Would have been about £75 if I hadn't got them to do the negative terminal as well.
Took them about an hour to find the issue and about 30 min to fix the main issue. Their hourly rate is £45. Quite reasonable I think, especially for such a good service.