Thanks for your help guys

all sorted now!
I evenchually found out what went wrong - when I disabled the internet sharing and re-enabling it, it was assigning a new I.P. address for network card on Pc that Xbox uses. Once I realised this all I had to do was using this new address in xbox and XBMP.
Thanks again - I guess those that lack brains cells (along with NTL) are me and Microsoft for thinking that NTL were capable of such a thing.......
BTW - has anyone else managed to get XLINK working as alternative to Xbox Live?