it can take around 6weeks when CT plates are involved, i take it the v5c you are waiting for in the one with the age related mark on it, i would ring dvla back give them them both registration mark's and then tell them that you want this sorted imediatley due to the fact you are selling the vehicle, if you shout loud enough it will go to supervisor call and they will have to sort it.
it could be that cardiff still have your paperwork but most probably it is in the cherished transfer section at swansea waiting to be done