The final solution at last...
Oooooh Kaaay...everything is now copacetic but I've learned one thing that I may as well pass on - ZoneAlarm and routers-on-a-switch don't mix well. I kept getting all sorts of puzzling and inconsistent results, fine one minute and then not the next. Eventually started getting "Ping: Transmit Failed, Error Code 65" error messages which prompted me to search Google (I'd never seen anthing like it before). Most seemed to implicate ZoneAlarm - removed it and all suddenly became smooth sailing.
So the lesson is, as RTFM intimated in his last post, "However, I've never been convinced about the merits of trying to "do" security on top off general purpose operating systems -- too easy for a flaw in the underlying OS to invlidate your security software. Coupled with the problems many people seem to report in using Zone Alarm, my personal view would be to learn how to use your dedicated firewall first...." So I'm off to learn about ipwadfm and ipchains and so on and so forth... And I've removed IPX/SPX...trust and improve your firewall rather than kludge it up. ZoneAlarm seems to be just fine with a direct dialup connection - suggest caution with any other arrangement.
Thanks everyone - it's been a learning experience for me and hopefully any PPRuNers with the same problem will find the answers useful