Thank you to you both. I've just re-set the RF out channel number, retuned the bedroom TV and my Sky picture is clear again!

It probably was something to do with the Freeview channels changing - in my area (Anglia) the digital switchover takes place in November and I think they're doing some preparation work for that. Round about the same time my Sky picture went fuzzy, I lost access to several Freeview channels including Sky News and Dave (when I googled this I found they're all on Multiplex C). However, when I changed the Sky RF out number, and re-tuned the bedroom TV, these lost channels came back. Thanks again for your help.