My Garmin 800 seems to have developed fault this week and as such the OS mapping won’t load up any more.
I’ve tried Googling it but can’t seem to find a fix for it so it looks like it might be the end of the road for it as I use the maps a fair bit.

Is there anything worth looking at apart from another Edge 800 or an 810?

Have you tried a different SD card? Or load some OSM mapping and try that?

If you dont need the training features the touring is worth a look

Have you tried a different SD card?

I never thought of that.
I’ll have a dig around and see if I’ve got one kicking about somewhere.

If you dont need the training features the touring is worth a look

I need the training features to work with a powermeter so it’ll need to work with that as well.

I know little of your particular garmin, but had a error with my garmin 310xt which I struggled with, a hard reset seemed to solve all ? Just s thought

Why not try the Garmin Edge forum they’re usually quite a pleasant bunch (unlike here).
https://forums.garmin.com/forumdisplay.php?245-Edge-800

PS My old 800 (now my son’s) wouldn’t load the maps last week, taking the card out wiping it off and putting it back in again sorted the problem.