Clearing cache and using Chrome instead still failed.
Waiting a few hours was the answer. And, when it failed again, wait ...
Using the VIP list is much better than BT Call Protect - where you have to receive a scam call first before you can block it.
I would recommend this route to avoiding scam calls permanently especially if you happen to manage your elderly parents' phone for them. I live 250 miles away, but manage it all via POA.
I also had 1471 ringback disabled on the line too - this stopped my mum calling the scammers back - usually didn't work coz they are spoof numbers anyway - but some did get through to a scammer - that was worrying at the time.