Thanks @gofaster. I've had a good read through that thread. I'll need to ask my housemates, who also use the service. Would a Chromecast be identifiable on the 'connected devices' page of the Hub One? There are several devices on there which tell you what they are, and others which are 'unknown'.
To see if I can fix this issue, I've moved my router to be within a metre of the BT socket (ie. no BT extension leads), and so far I haven't noticed any disconnects. But it's only been a few hours. I still have what I regard as terrible speeds for a fibre line which should be able to manage over 60Mb/s, according to BT. I'm getting about 18Mb/s down and 5Mb/s up, according to Ookla and the BT checker. The best I've had on this line with the fibre connection is 33Mb/s, but that only lasted a few days before dropping to a figure that was only marginally better than the ADSL line I had before.
