If the Sky decoder is connected (I assume) via HMDI be aware that it is also trying to upscale (depending on the setting for HDMI output to TV) and it may worse than TV upscaler.

I did set my HDD recorder to output at 576i on HDMI to my FullHD W805a and the image is far better than setting the recorder to output at 1080p..

Obviously in thi way you lose quality on HD image (that my HDD recorder is not capable to manage), but you may try some setting in the middle (I don't think SKY is sending any pure 1080p signal, 1080i if you're lucky)