Thanks for your reply. After a lot of head-scratching, I realised that my router reset had failed - the reason being that I'd screenshot the on-line instructions for an older version of my hub. I should have known earlier when I noticed that my WiFi network name and password hadn't changed. 😤
I've done the proper rest for my Hub 3.0 and things have happily returned to normal - my son's high speeds have returned.