Very few
Sky boxes come with s-video ... I think only certain Grundig ones and
Sky+ ones (if there isn't an s-video connector on the box, I'm 99% sure you won't get one through scart). If s-video isn't an option, I think all
Sky boxes come with RGB out and a £35 box from Maplin will give you an s-video signal from this (though I've heard both good and bad stories about picture quality).
Yeah, the SDI mod only works with the Pace 3000 box I think - I picked on up on
Ebay
for £50, the mod was £110 and the capture card £130 ... if you are good with a soldering iron, this is all you need ... if you pay someone else, they'll most likely charge you a few hundred quid! It's worth asking local repair shops if they'll do it ... it's not hard, just fiddly (make sure you don't consume alcohol beforehand!).
Al