Mark W

Thanks guys. It appears, if I export it as a "Siril built in sRGB" profile (which I hadn't looked at in the drop down box), it saves (or exports I should say) properly. I don't know why because I have no idea what any of this jargon means, I just wish to use the app but apps are developed by people who seem to think we all like to research every single tech/geek aspect about saving files and doing coding etc.
Anyhow, thanks again. Appreciated. If you hadn't prompted, I'd have been no further forward.

Thanks Walt, I'll have to take another look at what exactly, I'm doing then. I'm very new to Affinity (and processing in general). I'll work on it and see if I can identify the issue (sounds like it could revolve around what you suggest but I need to check). If it doesn't resolve, I'll return and give a full breakdown on the 'workflow'. If it does resolve it, I'll let you know.

I see the same in Photos on my iPad. However, in Affinity Photo it is correct, so I think perhaps you should try some other photo viewers.
I'm a little unclear on your workflow. You said when it looks good on screen, you go into the Develop Persona and develop it. That would only be needed for a RAW image, and if you had a RAW image you wouldn't need to "go into" the Develop Persona, as you'd already be there. Some more details on your workflow might help.
But I have a guess: whatever you're doing, you're either Saving a TIFF file that you Opened (which will produce a TIFF file with Zip compression) or you're Exporting to TIFF, and leaving Zip compression enabled in the Export Options. And the viewing programs you've tried so far may not like that compression. I'll do some more checking when I'm at my Windows computer, later.
Edit: Or perhaps they don't understand EDR images?

Both screenshot and tiff look almost identical to me - putting aside expectable differences due to rgb/8 vs rgb/32 and different resolution.
so no idea why / what you rate as „total mess“. Can you elaborate in more detail in what exactly is bothering you?