I've uploaded a video to better show the problem:
https://vimeo.com/791901404
A couple of updates, the jungle IC is a TB1230N, also I've brought up an inbuilt test screen from the service menu, and that shows the same problem, whereas OSD over teletext doesn't
I've got the datasheet for it, but it doesnt seem to explain how the OSD is overlaid over the source RGB signal... is blanking involved here?
Someone who used to repair CRTs is coming to take a look later this week hopefully, worst case I'll be able to get some photos if he struggles to fix it!