I'd always wanted to get all of the lights out and see what kind of caked on old crap I could find in there. I wasn't disappointed.
I removed the headlights from their housing, cleaned up the cavity and then gave it a coat of really good wax to try and prevent huge build up in the future. The headlight guides corrode easily, but a bit of light rubbing brings them up nicely. They're very easy and inexpensive to replace if you have to, but I didn't think it was necessary.
Headlight removed - clean up and wax in progress
Next I took out the lower fog lights and indicators. There was a tonne of crap behind here and the cables needed a good clean - making them look like new.
Another thick coat of wax in here...
Rear lights and reflector removed, and now the real cleaning began. I used ordinary car cleaner for a lot of it (having taped all the electrical bits and pieces up to protect from water ingress)
Then another coat of wax and a bit of elbow grease to bring out the shine.
Finally, I used autoglym fast glass on the rear lights and reflector (as well as all of the front lights), back to black on the plastic bits, and put the whole lot back together.