is the fog light button working ?

try the diagnostics screen thing just to see if that works at least it will confirm the button works .. nothing to do with the actual light as such but a good test

with engine running hold left stalk button in and turn fog on / off / on / off

you should see info come up on the little dash screen

(and is there a little led on the button itself ? I can't remember)

Thanks, I'll give it a go. The fog light button is working as when it's pressed the picture of the fog light comes up on the screen.

ah ok well that's a good start then I suppose. forgot about the warning light good point

is it fused ? At the risk of stating the obvious you've probably already checked

It's not separately fused and all other lights are working.
My mobile mechanic isn't an electrical fault lover so if he can't source the issue I'll have to get an autoelectrician out

Definitely worth reading through this thread re the fog lamp - I seem to recall a few lighting issues over the years: https://www.volvoforums.org.uk/showthread.php?t=102900

Turns out it was actually missing the relay ?
One ordered and broken wire fixed for the 3rd brake light.
Drop links fitted, having a new drive shaft fitted Friday.
Spending all this money on my sons car he doesn't want but I'm too deep to sell without the work being done. Oh well, someone will be buying a well sorted v70 when it's done

Now for sale