Do a Google search for
"fire caused by mirror" - it's not as rare as you might think! You don't need a well-figured mirror to reach combustion temperatures at the focus of a 200 - 250 mm make-up mirror. (If yours won't do it, it might be exceptionally poorly figured, or just a bit too small. My 200 mm mirror will make a solar image that is about 5 - 8 mm across, and trust me, you won't leave your bare hand there for long!)
The house referenced in the OP was well-alight at 6:00 pm, so could have been smouldering from 5:00 pm or so. Devon is 3 1/2 degrees west of London, and UK is on daylight-saving time, so local sunset is around 8:20 pm right now. At 5:00 pm local time say, the Sun would be at an altitude of about 25 degrees - I think I could light a fire with a make-up mirror in those circumstances.
https://www.bristolpost.co.uk/news/u...lieved-2784581
https://www.youtube.com/watch?v=v3gHAj_bE1M
Early morning in April in the UK - PVC window frame smouldering / smoking from a make-up mirror.
I maintain that the story is plausible, and unless you want to find out the hard way, don't leave your make-up mirrors where the sun can hit them while unattended.