Damn, what a pain in the arse !
There are several ways of doing it, the best is probably with a router, however as things are financially tight at present I decided to go down the "how many times will I swear before it works route ? "
This was because my modem is usb, and didn't really fancy forking out for a new router/modem with ethernet connections etc.
To cut a very long story short, with perseverance it will work ! Luckily my son has the new slimline PS2 with a network adapter built in (the older PS2 requires an add-on adapter), so I purchased a cross-over cat 5 cable and voila......bridging my existing wireless network and this one together and a bit of tinkering with my firewall settings to allow certain ports to open (Sony have this DNAS checker thing built in to every game), my PS2 is now online using internet connection sharing for a mere £19.99