I finally bumbled into my own answer:
I've been using 'NetRouteView' which is a free downloadable GUI utility to simplify access to the routing table and avoid using the ROUTE command in a cmd window.
- Add the second gateway in the IP4 advanced properties of the network card. Here they are on my local subnet
192.168.1.254 (original landline router - unlimited use)
and
192.168.1.253 (new, limited volume 4G router)
Each gate-way will add some lines to the routing table, for instance to handle low priority priority packets (I think) that look ike this:
Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.1.253 192.168.1.104 266
0.0.0.0 0.0.0.0 192.168.1.254 192.168.1.104 210
(My local PC is on port 192.168.1.104)
- Now we need to add a line to direct our (priority) VPN packets via the 253 gateway
nnn.nnn.nnn.nnn 255.255.255.255 192.168.1.253 192.168.1.104 11
nnn.nnn.nnn.nnn IP address of your remote VPN server
255.255.255.255 makes the route specific to this single address.
That works for me. The only issue I now have is that after a reboot or waking from sleep the nnn line changes the gateway to the 254 address and I have to delete and re-enter it. If I edit it, NetRouteView complains 'Element not found'. I have the line set as 'persistent or 'Don't Change' but the problem, rather than the line, persists.