An update. Postcastserver appears not to work via the VMC GPRS card, although it works fine over the LAN/cable connection. It talks to the Vodafone software OK (after I'd configured it to look for the VMC mobile connection when the LAN wasn't present), but then fails to resolve email addresses via DNS (according to the Postcastserver logs). It may be that I can manually set a different DNS server IP within Postcastserver to overcome this, but presumably I'd need to do some more digging about to find the Vodafone one (it's not obvious from the dreadful Vodafone support info).
For now I'll stick with your suggestion BEagle. I discovered that I could get the "old" Outlook send/receive toolbar button back in Outlook XP (the one that lets you choose which account to use), and this seems to work pretty well. It's just a shame that I have to manually check accounts and can't just let Outlook do it in the background.
The massive charges in France were "connection fees" rather than GPRS data tariff charges it seems. I was in a low signal area and consequently the VMC card kept repeatedly making new connections every time there was a momentary signal dropout. Every time it did so I was charged the "minimum call fee" or some such, all without me knowing! It's about time someone sorted these unscrupulous rogues out.
Thanks for the help folks, much appreciated.