I would have thought OS 10 would only run on Macs.
The problem appeared to have started while visiting theguardian, but went on other sites and followed when switching from Safari to Firefox. At one point I interacted to see what was going to happen, clicking on various questions but not of course on claiming my supposed gift, and was not asked for any data.
According to someone on Apple community forum, it"s malware.
The problem suddenly stopped and did not return.
By that time I had informed theguardian (received no reply) and clicked on malwarebytes'