What make of box do you have and what is the contrast setting on, i.e. Low, Medium or High?
If you have an Amstrad whos contrast setting is stuck on High, then matches on Sunny Days are going to be unwatchable due to clipping take place in the image.
If you have any other make of box whos contrast control works correctly, then try switching to Low when watching such content and it should help matters greatly.
Some are of the opinion that Low is the correct setting for a Sky HD receiver anyway, as they see Medium as clipping white, so try Low and see if you are of that opinion.
I have a feeling you were using Medium all along.
Also, make sure the contrast setting on your TV isn't excessive, as this may also contribute to the problem you're experiencing.