Is the picture the same from all of your input sources, ie, Sky Box, DVD Player, Freeview etc? If it is the same for all of your input sources then I think you need to adjust the picture geometry. Assuming it is a flat panel LCD or Plasma you should be able to do this from the menus, however, some manufacturers may hide this option on the service menu.
Have you got the TV manual, if the adjustment setting is available it should be in the manual, it is the vertical picture shift adjustment that should correct the problem.
Also, have you checked the output settings from your input sources using their menus. Sky boxes output in 16:9 or 4:3 which is selected from the Sky Box menu structure. Perhaps checking and resetting these might also fix the problem.
Rgds
Jack