Sounds like you have been a busy man mate. Looking forward to seeing this at a few shows this year! 
Spending more time on stupid little faults now than I have with anything else!
Cr@ppy pictureless update
Had trouble trying to use the exisiting horn wiring as I cannot get a positve test through the wires under the engine bay. Tested the steering wheel switch, that's fine, tested the sender wire to the fuse box with earh on the boss, passed. But when I put the testers earth on the car body and the positive to the horn switch sender wire, nothing. After lots of head scratching it turns out the steering column and rack is not earthed so will not close a circuit. Had a peek down the back of engine bay and I spotted that earth point 16 (rack to body) is broken.. Hope to sort this at the weekend!
After a few moments with the engine running, the Handbrake/Fluid Warning Light would come on and stay on.... I have now found the plug for the brake fluid level sensor (was wedged under the airbox), I had to chop the plug off the loom and make new individual connections to use it. The cap is not the orginal as the car has a mk2 16v servo etc.. However the cap isn't off a MK2 and have no idea what it's from!
I have tested the fluid cap sensor and it appears to be switched on all the time, so will need to see if I can fix or replace.
My other problem is that I cannot for the life of me find where the handbrake switch wire is supposed to go. Having a connector at the end of the wire suggests that it shares a connection with a low fluid level sensor wire away from the fuse box, I cannot see any spare connections. I checked the following pin locations at the back of the fuse box to try and trace somewhere to stick it.
C/12 - brake fluid switch
B/1 and A/15 - brake fluid/handbrake warning switch
E/13 - handbrake warning switch
In my new looms there are no wires in the above pin locations! WTF!? Looks like there never was.
Resorted to plan b, removed instrument panels and located plug for the warning light, found the correct earth wire and spliced the handbrake switch to the plug, did a continuity test and lifted the handbrake, result. however, when I put the battery back on, I heard a fuse pop.. now obviously the switch does nothing again.
