I don't think the Sky HD box on its own could interfere with the wireless signal (although I just have Sky Plus so can't be entirely sure if HD has any extra capabilities). The AV sender unit I referred to is made by Fergusson and plugs into a Scart socket, sending the AV signal wirelessly to a receiver upstairs. The channel allocation is made by pressing a button on both the tx and rx so that the channel numbers match.
There are so many things nowadays that could interfere with the signal, so I'm sure it would be worth asking the neighbours if they have recently bought anything new. Just to eliminate the Sky HD box as the problem, switch it all off and see if it makes a difference.
In my house I have a cordless digital phone, AV sender, weather station with remote unit, and up until a couple of year ago a baby monitor, all of which I guess could interfere with each other. If you have anything similar try switching it all off.
Regards,
Vindaloo