Thanx to everyone who responded on this... I'm happy to report that the problem is now resolved. There turned out to be two things I needed to do in order to resolve the problem. Firstly I needed to be using the same network/netmask pair as the "official" network (I had been mistakenly trying to work from what ifconfig reported after establishing the ppp connection). Secondly, I needed to add "gw remote-ip-address" to the "route add -net" command.
Thanx again, it's working beautifully now!
