Not the only issue today
Woke up this morning to my Plusnet hub rebooting continuously and hence no internet. Reported it as a fault then discovered it was a Chromecast Audio update in the early hours causing the problem.
https://productforums.google.com/forum/#!topic/chromecast/3PiH_40qLBw
Not saying it's a Plusnet/BT router issue but neither company appears to be acknowledging fault at the moment.