You don't use Open DNS?

Try it .. it works.

https://www.opendns.com/start/

Just make sure you record your previous settings if it is not set to automatic.

Hi Lindsay, I was aware of this workaround, but didn't feel I should really have to.

However, I'm truly fed up with connectivity grinding to a halt at random, so I have made the switch, and will monitor for a few days, to see if it cures it.

If so, I think we'll have more-or-less proved it's a Plusnet problem. If not, I don't know what to try next...