Well I seem to have solved the Plusnet monitoring issue when I changed the default gateway to one where all gateways are tier 1.
For some reason when I had it set to the gateway group that has both DSL as tier 1 and LTE as tier 2, that wasn't working correctly. It blocked anything specifically directed out of the Plusnet WAN from the pfSense box, plus never fell back to LTE even when both modems were switched off. Discovered that yesterday when my drop wire was being replaced.
Its really bizarre as it makes no sense that setting it to the gateway group where everything is tier 1 would make a difference, especially as monitoring WAS working for LTE which is what you'd think "might" break when it was set to tier 2. I wonder if just changing the default gateway has somehow fixed a configuration glitch.
Can't say I've noticed any problems with Unbound though.