Well, haven't been in here for a while so decided to do a bit of an update. David (WARFARIN) sorted me out perfectly with a set of second hand light units, so the next task was working out how to fit them and wire them all up properly. Basically this is what I did:
Following a wiring PDF i found online, I wired the brake/indicator wire across to the middle lights (running amber bulbs).
Then I ran a separate wire from the brake pedal switch to the back, and hooked that up to the outside lights.
Then, I drilled a small-ish hole in the back of one of the middle light assemblies, and fit a 55w halogen in there (there wasn't much room for another bulb, so a halogen was it, due to it being so small). I rigged up a relay, ran a wire to the battery and ran the switch to the reverse wire. This became my new reverse light. I realised that I could have a heat issue if I left reverse on for too long, but luckily reverse is something you are only ever in for about a minute at most, so wasn't so worried about it getting a little warm in the plastic assembly. Doing it this way also meant I didn't need to hook up a separate external light, which I felt could possibly alter the nice aesthetics of the car, and would be more stuffing around drilling holes and running wiring around the place.
So there you have it. All nice and legal now. Thanks again to David for the lights! If anyone wants any pics of the setup, let me know.
Matt