Today my mum and dad got a 5.1 surround system, and its working fine on DVD obviously but what do I connect to the optical output to get the sound workin okay on SkyHD?

The Sky optical output needs to connect to an optical input.

If you have bought an amp you should be ok for digital inputs ,but if you have bought an all in one then you may have a problem as most dont cater for additional devices in Dolby Digital.

If the back of the system has no optical input you're stuffed unless

it has a coaxial digital input then you can buy a converter to take the Sky optical to the amp coaxial