This guide is for anyone who already has a fob programmed to their car and wants a spare. I'm crap with soldering but I managed it, so just about anyone should be able to..
Small phillips screwdriver
Soldering iron and solder
Spare fob (got mine off ebay for £3.00)
First thing is to undo the small screw on the back of each fob and get the circuit boards out.
What you need to do is remove the blobs of solder on the new board that are different and make a copy the board that's for the car.
Use the soldering iron to heat the existing solder and then use the desoldering tool to suck them off (fnar) once they melt
Continue until any parts that are different are removed.
Then add some new solder to replicate the existing fob
Obviously I'm not going to show you the board from my car..
This took me about half an hour, start to finish and I now have two working fobs for my car. Hopefully it'll help someone else out!