OK, I've made some good progress; Starlink configured on WAN1, and PlusNet on WAN2; with the latter on Fallover Only, when I unplug the Starlink feed after a minute or two PlusNet kicks in.
However, whether WAN2 is set for Fallover Only or 10% Load Balancing, with Starlink connected on WAN1 access to my local Home Assistance NUC via DuckDNS doesn't work.
How might I get around this? Inbound access to my LAN is important for me. Is there a paid-for VPN type service that can cope with non-static IP addresses?
EDITED TO ADD: This chap seems to think that putting PlusNet PPPoE on WAN1 and Starlink DHCP on WAN2, and then setting WAN2 LB to 99% all should work.
EDITED TO ADD #2: I have now switched things around, with PlusNet PPPoE on WAN1 and Starlink DHCP on WAN2, with WAN2 LB set to 95%. Speedtest.net results show an improvement over what I would expect from PlusNet and my DuckDNS tunnel in to my Home Assistant NUC seems to work also. Great news. Next task is to try to get access to the Starlink control panel which needs some more reconfiguring ...