As
@Landmark2 said they are indeed E10. I intended to buy one from Halfords for about £4 but thought I would just try Euro Car Parts on the way past on the off-chance and managed to buy one of these sets for less than £10:
https://amtechdiy.com/product/10pc-female-torx-socket-set/
So, I marked the position of the plastic plate on the scuttle with a pen so as not to change the headlamp alignment, and removed the light unit without any further problem. I took out the bulb and held it up to the light. It looked okay but I tested it for continuity and it was okay. The contacts all looked clean but gave them a squirt of contact cleaner to be sure and put the bulb back. Nothing. Took it out, put it back and it worked and nothing we did would stop it working. So, problem solved for the moment at any rate.
However, as is the way with this car, sorting one problem creates others. Firstly the cover was missing from the main beam light. Instead it was covered by a scrappy bit of duct tape. Second thing I noticed was water and muck inside the clear cover. Perhaps not surprising given the lack of a cover but although I knew it had condensation sometimes I hadn't realised how much water was in there. Presumably it normally all sits at the bottom and I don't notice it. Finally the dipped beam reflector was cracked. So, two options - 1) buy a replacement headlight unit if I can get one or 2) replace the cover (XBQ000050 – £26.87 from LR), remove the clear cover, dry things out, clean the inside and ignore the cracked lens.
The key question for option 2 is, can the clear cover be removed? Has anyone done this or know of it being done?
.... i'm affraid you'll have to drill it out then use a rivnut or something similar
