So here's a quick fix that seems to work:

Allocate a gap in the DHCP scope which will be dedicated to VPN clients. Then set Routing and Remote Access to use the allocated range of IPs. Here's how:
1. Open Routing and Remote Access MMC
2. Right click on the server  and select Properties
3. In the Server Properties page, select the IP tab
4. Instead of using DHCP, select Static Address Pool, then define the addresses which you've excluded from the DHCP server.

Hopefully this will help somebody else out there!