Ok that's embarrassing I suddenly realised what I needed to do and it looks like it works.
Added a static route for the VPN server IP address with default route "lte1" (my wan interface) with route distance 1.
Added a default route for the VPN with a distance of 2.
Increased the default route distance of "lte1" to 3.
Now when I trace route bbc.co.uk it goes through the VPN ... Happy Days!
If this is the 'wrong' way to do this or if there is a better way please let me know.