"Tim S" wrote in message
news

So I thought about using either Celotex/Kingspan to 50mm, leaving 50mm
breathing space in the rafters under the tiles - or rockwool slabs.

I have done same with Celotex - roof and walls behind plasterboard. It
insulates superbly (though you need a bit more than 50mm to meet latest
building regs if you have to) but it is a doddle to fit. Yes it does give
off fumes when ignited but does not sustain the flame on its own so it's as
safe as most other materials in the house.

If it isn't a new build contact "Seconds and Co" (use google) who supply
rejects - usually a mm or two too thin but a lot cheaper.