In the British Isles, the main reception problem is usually rain - unlike a sunnier zone like Southern Spain, where, because it's right on the edge of reception area (or outside) you can have problems as the atmosphere heats up in daytime as well as in rain.
I'd also be looking for local possibilities (like interference, coax lead).
I did think of one other, which your symptoms don't make me think is the problem - I did find I lost a whole bunch of transponders, and that was a fault in my satrec. I confirmed it by borrowing one from a neighbour (he had a spare, luckily for me). The effect was as though the H/V switching just wasn't happening.
I haven't looked to see whether the Sky channel is near the Hi/Lo boundary, which would be another possibility.