Sounds like malware, Jux, or something very close to it. I would do a full scan with your AV suite, but also download Malwarebytes for reinforcements (free). www.malwarebytes.org/
The idea is that if your AV suite doesn't find it, Malwarebytes will. If neither finds it, try Superantispyware! I've found these two to be very effective, though others might have good suggestions too
Thanks. I have a pop up blocker and ad blocker - the usual ones, recommended on MN years and years ago and I have been installing them ever since on whatever machine I have! For MN I tend to use an iPad.
I shall get the other things suggested and sort it out properly. Tbh I don't use the laptop much, it's back up really, for when I'm stuck in bed and need to run programmes for the course I'm doing, so don't usually go on internet on it at all.