A bit of googling threw up a rather useful piece of software: Hamachi. Now run by the folk at LogMeIn, Hamachi is a cloud-based UDP VPN that needs very little configuration. All you need to do is install the client software (available for Windows, OS X and Linux) on all the machines you want to connect to the network. These build UDP tunnels to the Hamachi servers. You can then define a network name (and password), and your systems can then be connected to each other through your new network. There's a free version which runs as an application, and a premium paid version that can be run as a service. The free download gives you 30 days of premium service - a good way for LogMeIn to add paying customers!
Sensibly the Hamchi servers issue IP addresses from the 5.x.x.x range, so there's very little chance of colliding with IP addresses on the local network (something that happens all too often with traditional VPNs which try to map one network's structure onto another's...).
It's not quite the promised zero-configuration VPN - I did find that I needed to open up one set of UDP ports on my home network firewall, but everything else ran smoothly, and I'm back with email at last...
Another tool for the mobile computing toolbox.