trt I don't know how old you are but, at present, it is free when you reach 70. The following was copied from the Direct Gov website:
"You’ll need to:
fill in the photo renewal form D798
include a new passport type photo of yourself which must be recent and a true likeness - you don’t need the back of the photo signed
return both the photocard and counterpart of your licence – if you have lost one or both parts you can still use this application but will need to tick the relevant box in section one of the D798
enclose a cheque or postal order for £20.00 – no fee is needed if you have medical short period licence or you’re aged 70 or over
send your application and fee (if applicable) to DVLA, Swansea SA99 1DH
If you have changed your name, you’ll also need to provide identity documents as proof."