Alex,
I've not really much idea what the GAPAN tests are, but a quick google tells me that they are the "Guild of Air Pilots and Air Navigators" tests, and are held at Cranwell (someone jump in and tell me if I'm wrong!)
If this is the case, when I took the Airmen Aircrew tests at OASC in 2002, I found that they also quoted my results from my 6th form scholarship tests (Pilot/Nav) from the early 90s.
When I queried this, they told me that the results were valid for 10 years, effectively meaning that if I had failed them on the subsequent attempt, the previous results would have stood (again someone correct me if I've been given duff gen!)
I'm not too sure about how many attempts at the tests you are allowed in what time frame, but something about 2 attempts rings a bell with me. The best advice I could offer is ask the AFCO or OASC themselves on the current policy.
Hope that is of some help, and apologies if I'm completely off the mark! Good luck whatever happens!