1X new HDTV with one scart 1X Sky+ box 1X older DVD player with speaker system (no Scart connection, RCA cables only, though does have optical cable port)
And I am hoping the end product will also enable Sky and DVD sound to be output via the speakers.
You really should read the owner's manual for the Skybox. The Scart connections on the Skybox+ are outputs not inputs. You do understand what that means, don't you? You cannot connect video/audio INTO them which apparently is what you are thinking of doing with your DVD player.
Your stereo system only have one set of audio inputs?
If your stereo receiver has only one set of audio inputs, then it is time to upgrade.
Scart has both audio and video. So, if you are using that to connect the Skybox+ to the TV, your TV speakers will be outputting the sound. Since you want audio from your stereo, connect Skybox+ RF1 out or RF2 out to TV's RF in and connect Skybox+ audio out (red+white) to one set of audio inputs on your stereo receiver.
Next, connect the DVD player's audio out (red+white) to the second set of audio inputs on your stereo receiver and connect DVD player's composite video out (yellow) or component video out (red,green,blue) to the TV.