Contrary to the previous answer, satellite signals ARE adversely affected by the weather. Very heavy cloud cover and especially rain can cause exactly your problem if the signal is marginal, for example due to a badly aligned dish. Trees and other semi-solid and solid objects in the way will also block the signal.
It sound to me as though your dish may not have been aligned very well from day one.
As it has been problem from day one you MAY be able to get Sky to rectify it for free but they only have your word that it has never performed satisfactorily. You would have had a claim under Supply Of Goods and Services Act but under the Act, you have to report a fault as soon as possible - which you have not done.
I can only suggest that you call a local independent Aerial Installer and get a quote for realigning you dish.
T
October 2009