Update:

I know how annoying such vague questions can be, i'm just hoping some of these symptoms are signs of a common fault I can fix quickly.

After my last post in this thread I have been away from the car and unable to check Earth wires. I can't think where I would start with that...any suggestions for major/important Earths to check? Earths for the door-open warning light (near the door hinges)?

After driving the car for the last couple of days:

*The horn is now completely redundant. When I press it I can hear a clicking, like in a relay, down by my knee (while in the drivers seat).

*I've noticed none of the door check-lights come on if you open the doors while the car is running. Although if you open/close doors while the cars performing the check, you get a mess of messages from the check-lights.

*Upon starting the car on a couple of occasions, I noticed the fuel gauge needle was resting in the UP position (not DOWN like normal). After the car started it moved down to where the fuel level actually was.

*Maybe unrelated? There is a liquid very slowly speeping from behind where the fuel needle would be on a stalk. It looks oily, so i'm not sure how this would affect conductivity.