Happy to report: the frequent disconnection issue on my end was internal and not related to the broadband service.
I can also confirm that the Plusnet router as sent with the standard broadband package is highly problematic if you connect to it through ethernet rather than wifi. On investigating poor connections (e.g. really erratic quality on video chats with long pauses) I noticed pings were often > 1000 ms whenever more than one device was connected, and during uploads the internet was unusable on other devices/for other connections (e.g. as DNS services cannot be reached). This has previously been reported in Plusnet forums as 'bufferbloat'. The problem was entirely resolved by replacing the modem-router, confirming it is a Plusnet router issue, rather than what is usually meant by bufferbloat.
Unfortunately my first replacement modem-router, though fully resolving the ping and 'congestion' issue, the Netgear AC1900 D7000 had its own flaw (unknown to me) of intermittent disconnections from the internet addressed only by restarting the unit. With an alternative modem (Draytek Vigor 130), I know have a 'normal' internet connection.
