R/T license is a seperate document, yes.
You can apply for this seperately to your main license, but there *is* a charge of £70 from the CAA to do this, see page 2 of this doc:
http://www.caa.co.uk/docs/175/srg_fc..._ppl_09_10.pdf
If the R/T application is combined with the PPL application there is no additional cost.