The rescue centre phoned me to say they had been neutered and come round after the op, and to say the vet had told them the cats are a pair of females, not the male and female we thought! I have to admit I'm slightly taken aback that the sanctuary hadn't checked. It makes no difference to us (I was very surprised when the lady asked if we still wanted them), but would have thought it would be routine to establish the sex when rescuing cats. Apparently they came via another organisation who had handed them over as brother and sister.
I'm curious to know if there will be any difference having a pair of girls as opposed to one of each.