So, to recap, I installed Adblock on 12th August, along with my 'donation' of US$20.
Unnecessary that donation may have been, but since I made that post on 12th August I have not seen "website not available" once, nor any adverts on Pprune or any other website.
Coincidence? Maybe. But I think it was excellent advice; many thanks to Booglebox.
And I think the developer deserves to be rewarded.