OK I've managed to get it to work now..I still dont fully understand the changes in sql2005 but here goes for the sake of others who might find themselves in the same position.
SQL2005 by default doesnt listen on TCP only local traffic and named pipes, which might catch you out on first installing it.
SQL2005 has a defualt port of 1433 same as sql2000 which of course causes problems if you have an installation of sql2000 on the same server because of course both services cant be listening on the same port! Sql2005 takes precendence in my installation but I dont know if thats always the case.
SQL2005 has an extra service on port 1434 the SQL browser service - I initially set my instance of sql2000 to listen on port 1434 so of course that didnt work either!
Once I set up the sql2000 instance to listen on an unused port this form of connection string worked ok.
"Provider=SQLOLEDB; Server=18.104.22.168\instance,port; Initial Catalog=ITBuy; User ID=SilverSurferAccess; Password=G1ll1anD1ane; Language=BRITISH ENGLISH"
Thanks for the help,