I had the same problem 3 mths ago when I went to add the FAA IR to my FAA PPL based on my JAA PPL which had subsequently become a JAA CPL.
It's free (unsurprisingly, apart from paying the CAA 15 quid) and pretty painless, also they were very nice and added my MEP rating on it too.
I worked out recently that I've paid approximately 3500 quid
to the CAA in exam/test fee's/medical and associated rating issue (aka printing out another piece of A4 with my details on it) since I started flying.
