I reset the HG612 back to factory, and connected it to the Plusnet Hub One as
Roseway suggested, and it does indeed make things feel snappier. The Google speed test graph tachometer immedietly springs up to ~38Mb with this setup, before it jerked upwards in a stop-start fashion (though still ultimately showed the same throughput).
A downside of this configuration is that the Plusnet Hub One is very locked down, so much so that you can't even change the DNS server
So I ended up configuring pi-hole ad-blocker to also be a DHCP server, which can offer my own choice of DNS server. This took a bit of fiddling, but worked well - and the Pi it's running on is being powered by the PlusNet Hub One USB port
Thanks to everyone for their help in getting this all working
Ian
ps This
https://www.grc.com/dns/benchmark.htm is an excellent Windows program for benchmarking DNS server speeds, to find the best one for you. Let it build a custom list (can take about 40 minutes) then run the benchmark on this list, and it'll show you which ones work reliably and fastest. I'm now using 1.1.1.1 from Cloudflare.