Hi The_Chop & welcome to the forums.
I'm pretty sure that PAL Gamecubes don't support S-Video cable connection, in the same way NTSC Gamecubes don't support RGB Scart.
As you're no doubt using a UK PAL console then I think it's just a case of an incompatible cable. I would suggest getting hold of an RGB Scart cable(providing you have a spare Scart socket in the back of your tv), should solve your problem and is also superior to S-Video in terms of picture quality
Hope that helps.