Roly wrote:
A friend of mine has some problems with the roof on her house. The
lining is ripped and hanging loose in places, some tiles are loose and
there is rot starting to appear in the timber under a valley part of the
gable.

Her instinct is to get the inside of the roof sprayed with foam, partly
to sort those problems and partly to further improve the insulation.

I've no idea if this is a good solution or what sort of questions to ask
a surveyor when they come to quote for the work.

Any advice or suggestions ?

Arrange for a relative to become urgently sick...and leave the area at once.