Thank-you both, have just got back from the trip (flew back SFO-LHR on Tuesday's departure this week).
Yep, was advised on checking in at LGW that the delay was caused by a bird-strike the previous week which had left them an aircraft short.
VS043 on 1st October itself departed slightly over four hours late, additional information from the flight deck that the knock-on delay from the bird-strike had been componded by our 747 ('Ruby Tuesday') requiring an air-conditioning unit change during the turn-round too.
Refreshment vouchers were issued and, apart from reducing the Vegas time of our trip by 4 hours, all was well.
Could have done without getting up at 0400 and then spending 7 hours at Gatwick but never mind (although I did receive an e-mail from Virgin at 0918 on 2nd October telling me that VS043 on the 1st had now been retimed to depart at 1445 and check-in now possible until 1345).
It seems there is no spare capacity in the fleet/service plan to cope with delays, which is a pity but not a surprise - it is very much the same in today's modern rail industry too, although the resulting delays to trains are usually (always?) far less than this.