Summary of where I'm now at. I had the DPF and pressure pipe replaced before Christmans, and yesterday had the Diesel Additive tank replenished with 1 litre of PatFluid. I also ensured the battery was healthy, using a CTEK Smart Charger. It was showing 80% charged in minutes and full charge in under 3 hours.
The car is running fine, seems more responsive and the fuel economy appears marginally improved. The only fly in the ointment is that the ESSR warning is still appearing. It occurred immediately after I picked the car up yesterday, so a quick return to the Garage saw their diagnostics (Snap On) indicate '6620 Glow Plug Control - Faulty Signal Temperature Too High'. All the other faults E010 (Additive) and 2A40 (Particle Trap Overloaded) had cleared.
At this stage I now intend just to run the car and see what happens. It starts fine, drives fine and the major concerns I had have now been addressed. The only question I now have is whether VIDA Vice devices would be more likely to clear the messages than the diagnostics my Garage have at their disposal.
I know Jason199 has a thread here
https://www.volvoforums.org.uk/showthread.php?t=276677
with similar issues to mine, but I don't want to hijack it with my own questions which don't address his concerns.
As usual, any advice welcome. Thanks.