OK, laptop is now connected directly to modem. It seems I had to tick the box labelled "allow all users to use this connection" or something similar.
Speedtest.net is reporting about 4.5 Mbps, rather than the usual ~30 Mbps
I've noticed another user is reporting similar problems in this thread:
After running another speedtest (wednesday, ~8am), the laptop connected directly through the modem is now at a much more usual 37 Mbps.
So how do I go about getting the hub replaced?
I hadn't noticed any speed problems, and a test I've run now (17:37) shows 37 Mbps. I'll do a few more tests through the evening, but the slow one yesterday might have been a one off glitch.