I did try moving a small magnet close to the screen (without touching it) those circled spots were changing but then going back to the same thing - while the blotches were changing colour.
I'll have a look at the magnets on the tube, I think they are safely taped in position.
Where would I get a de-gaussing coil these days? I saw one on aliexpress which runs on mains and I got shivers.
