It's certainly possible that the LNB (the 'electronic 'head') on your dish hasn't got quite the right amount of twist on it. Over the years, I have corrected this on a number of dishes and it's common to improve just those channels for which reception was previously at the limit.
It isn't really a question of a 'fault within your equipment' so much as a slight misalignment of your dish. The mechanisms behind digital reception mean that you still get decent reception on most channels with a small misalignment, but the weaker channels might suffer. If the problem gets worse when it's raining, this is probably the answer.
As for Sky's charges, I don't live in the UK any more, so unfortunately can't help.