Why did the GP think is was prostate cancer, and not, for example, some other cancer?
Has he been having periodic PSA tests (probably every 1-2 years)?
If so, what were the readings?
If not, get one done now - GP nurse should be able to do it.
I don't know if GP referral for reoccurrence is subject to same 2 week limit as for initial diagnosis, but you should push for that. That may be what GP meant by writing to oncology, but you might want to confirm.