Ah, thank you, that might well explain it. It may not be possible (or worth it) on PlusNet but I can move the folder I use for this sort of thing to my other domain on ICUK/CIX where I know I can get a certificate set up.Host it over https and they will load.
Browsers are getting fussier about accessing just http sites. It looks like Firefox is a bit more relaxed than Edge and Chrome but Safari is confused.I don't know what causes it but it's been happening a while on here but I know images over https fixes it.