Thanks for suggestions - they all helped. Good site that at
http://www2b.abc.net.au/science/techtalk/ 18-Wheeler - ta muchly. Eventually got it sussed and now trying to figure out exactly how. With the network up and running, once the ICS server was correctly set up on the 98SE machine (often a fiddle involving Regedit - see
http://support.microsoft.com/support.../q238/1/35.asp ) the XP-Pro eventually seemed to find and use it automatically!
Make sure that in Explorer/Tools/Internet Options/Connections/Local Area Network/Sharing is enabled and has the correct Dial-up Adapter and Network Connection identified (no choices unless you have nore than one NIC or modem)
I also had another modem (and connection) set up on the XP and the XP was using THAT to connect (on another phone line) in preference to the ICS connection in spite of my saying (in Internet Properties/Connections) "Dial whenever a network connection is not present" - I guess that the time that the ICS server take to dialup and connect is just too long for the XP which then gives up and dials the ISP in it's own modem. Somewhere in the Registry there will be a tweak that will make it wait longer before it gives up on the network connection.
Have to say that once M$'s 98/ICS is correctly setup on 98 (non trivial) it has always seemed to work well. Suggest care with AnalogX's "Proxy/Proxyi" (free) proxy server, which is arcane to get working (never succeeded) and once installed is virtually impossible to remove without major Registry restoration/surgery. Fobotcso - thanks, I was just about to try WinGate when ICS started working - sounds good (and free for <3 connections).
As an aside for other 56K/POTS users, by design the ICS client cannot disconnect the ICS server - but if you ever need\want to disconnect remotely then Twiga's Remote Disconnection Utility [which does a few other neat tricks too and is very professional], free from
http://www.twiga.ltd.uk/ works very nicely with this setup and is simple to install and configure - they have an updated rduserver.exe file that also recognises connections made with ISDN devices.