In case anyone is interested, this is now fixed. It was a damaged flame sensor cable. I noticed one of the dads at my sons cub scouts group turned up in a van proclaiming he was GSR, so although I didn't know him, took the plunge and showed him my video and instead of hanging around waiting for cubs to finish agreed to pop round and have a quick look.
It looks like when the BG engineer "repaired" the ignition cable he managed to twist the brittle flame sensor cable which was now making minimal contact. He said if I got myself a new flame/ignition cable he'd fit them, and since then the problem hasn't occurred once.
So lets recap:
Initial problem: Worn out ignition cable. £99 to BG to "repair" it by sticking it together with tape and damaging the flame sensor cable in the process.
Second visit, another £99, now we know it was a damaged flame sensor cable but BG went away advising a £685 powerflush and £3000 for a new boiler
£31 to source new ignition/flame rods myself and the promise of a few beers to my new mate for fitting them.
I think I'll be cancelling my BG cover from now on! (although I did get a new aquastat/ignition pcb/thermistor and gas valve worth over £600 for a £99 call last September so it was worth it in the long run).