I'm still not sure what the original fault was but I think I have narrowed down my current one.
I had a blown full beam indicator bulb and replaced it with an LED version in the process of repairing my loom. the LED is in a casing similar in shape to the original bulbs but its longer and doesn't seem to screw fully home without skewing off to the side. It could be causing a short to earth or perhaps the circuit is not liking having an LED in it. This is strange as I have read of people changing bulbs to LEDs with no problems.
I confirmed it was the LED by trying it in a different place as a dash light. This then caused the fuse to blow when the light switch was switched to park only.... Why this LED would cause this I have no idea. Any ideas?
Thanks
James