Hi Neil
The ENT guy at the local support group mentioned one possible help which you may not know about. Apparently they give all their patients a Neilmed Sinus Rise kit as it helps to clear out the rubbish. Using one is not a very pleasant experience but I found it did the trick for me. This link https://www.guysandstthomas.nhs.uk/resources/patient-information/pharmacy/neilmed-sinus-rinse.pdf has a description. I bought my kit from a local Boots so they are generally available.
Tim