Thanks NWIFlyer - that was what I had found out, was just checking I hadn't missed anything.
Thanks Cymro - unfortunately end of my TP year is 8th Jan and I can't imagine I could (easily) get another 10TPs for < £101 (once you take into account getting to and from LHR/LGW from central London, plus the time involved).
So, it's basically £900 for the two tickets, guaranteeing keeping my silver card, vs maybe £500 for standard economy flights, so it's decision time....