Got there! Added the alias interface on the RUT240 - 192.168.20.254, then added static routes on the RUT240 -
192.168.10.0 via 192.168.10.254
192.168.20.0 via 192.168.20.254
Then added a static route on the laptop -
192.168.20.0 via 192.168.10.254
The final static route is vital as it directs packets down the L2TP tunnel to the LTE router.
Thanks for making me think a little further.