Fortunately, while O2 have now updated the iPhone app to distinguish between Giffgaff (their own cut-price brand) and official O2 SIMs, merely inserting a free O2 PAYG SIM occasionally (each version upgrade, I think?) is apparently enough to keep the app active.

All the reseller customers got was a 10% bonus on topups applied in the week following the outage - not really much of a benefit!