Quote from a not-too reliable source (DM)
"He said the builders on the roof had been cutting materials and doing other work during the day. "
Eh-oh, those pesky angle grinder sparks again !, or was someone applying torch-on felt ?. This might explain all the black smoke.
Will 'managers' ever learn that when working on an old building especially when using soldering kit or angle grinders that a fire marshall with a suitable extinguisher should always be present at the immediate vicinity of the work.
You'd think after Uppark House went up in flames in 1989 they would have learnt their mistake, but no, it seems not.