I wondered if I'd changed a setting somewhere or installed something that had broken the internet. I was trying to think what I'd done on the computer a few days previously when the problem started. It seemed to be the computer because my Android phone managed an update of Opera of about 11MB in a few seconds while the computer was off- something that was taking an hour or so on the computer with frequent lost connections.
I was thinking about calling up my ISP to see if there was a problem when I thought I'd better check the physical connections first. What I found was the ADSL filter sitting on top of my laptop power supply. I'd punted it there while doing the hoovering a few days previously, and the EMR from the power supply was obviously interfering with the signal passing through the ADSL filter- explaining why my phone got a good connection when the laptop wasn't working, and why the problem had started a few days earlier.
It only goes to show that it pays to check the wiring before blaming the ISP.