Have you got another HD source (laptop, bluray,games console) which you could try - would at least tell you whether the problem is with the TV input or the Sky+ output ?
my experiences of Sky tell me that Sky themselves will not have a clue either.....other than to charge you for a call out so you can continue paying them to watch their programmes!!!!! As you can possibly tell I've had some bad experiences of Sky recently. If it wasn't for them monopolising sport coverage........