I have managed to get rid of the PUP by following instructions on this:
malwaretips.com/blogs/remove-potentially-unwanted-program/
which suggests using both Malwarebytes and a program called HitmanPro. I got a trial licence for the HitmanPro, ran it and the PUP has not reappeared after a reboot and 10 or 11 hours on-line. One interesting side effect of running Hitman is that my Malwarebytes Web Protection was disabled and I could not re-enable it with the slide switches. Malware bytes kept telling me to enable Web Protection (I have a Malware Bytes Premium licence, so it should work). However, when I rebooted the PC, Malwarebytes came up normally with Web Protection enabled.