Coswurv, there is nothing you can do but bite the bullet and buy a sky hd box. The reason the picture looks bad on your 50" TV is; the normal sky box only outputs a small number of pixels (blocks). Lets say 100,000 pixels makes up the image, this looks ok on a 20" TV as the 100,000 pixels fill this ok.

When the TV is a 50" these 100,000 pixels are increased in size to fill the screen. This is why you see blocks at times on your screen, probablky more on fast moving images. Sky HD uses more pixels and the picture looks sharper on larger sets such as your 50".

***This is a basic explanation of how this works and is obviously more complicated****

edited to add, Comet are doing a SKY HD deal for £99.