anyone please feel free to add to this or whatever, this is just the process i used and it seems to have come out well:
i used a can of savage tail light tinting spray (about £10-11) and a can of halfords clear laquer (about £4)
you just take out your rear lights and give them a scrub with soapy water, rinse them off and dry... then mask up the reflector with tape (otherwise your lights may not be legal). do this bit carefully and take your time - one sure way to make the job look crap is a wobbly reflector line
place the lights somewhere where stuff like hairs and bits of crap won't get blown onto them (like a garage for example) and spray very light coats of tinting spray leaving at least 10mins between coats. the thinner you do the coats, the better the overall finish will be, but make sure you get a good coverage with each coat.
once they are pretty much as dark as you want, leave them to dry and then trace the edge of your masking tape with a sharp blade so that when you peel it off it doesn't take any of the tint with it... then with the tape removed just do one final light coat paying attention to the reflector strip just to blend it in a bit (but only do a light coat as again the legality issue may arise)
once its all dry, start with the laquer using the same technique as the tint spray - light even coats, waiting between each. build up several layers until you are happy. and make sure you get the edges of the lights etc to seal it all up.
once its properly dry, give it a good t-cut to smooth off and then wax it.
sorted! et voila: