Yep, 2, both Thompsons. All 3 boxes exhibited the same symptoms described by nodnarb, and Sky had absolutely no answers, that much was clear. Still they stringed me along until the warranty was over then helpfully suggested that I pay for an engineer to come out - presumably so that I could be told that I needed to pay full price for a new box that they were giving new customers for £50!
Anyway, I got fed up and jacked it in and got a Humax freeview PVR instead. I'll miss HD broadcast TV, but the only thing sky-specific that I regularly watched was Sky-1 and I can download (in HD) anything that I would have watched on there.