You'll have to buy a HDMI to Scart converter - not just the cable. Scart is Analogue whilst HDMI is digital, so you can't just get a HDMI>Scart cable. Equally you can't just get a Scart to HDMI as the signal is not reversible (I believe).
So you'll need hdmi from the Sky box into a HDMI to Scart converter, then a scart from the converter to the TV.
http
/hdmitoscart.co.uk/