https://www.superquilt-insulation.co.uk ... x+10m+Roll - This is the YBS Superquilt, amazing stuff, very easy to use. Just staple it to the rafters, it acts as vapour barrier to. I put earthwool inbetween the rafters but tbh it was not needed with the superquilt and celotex. Leaving the gaps in the rafters will allow the roof to breathe.
https://insulation4less.co.uk/products/ ... 7409001525 - This is the place i use for insulation, 50mm should do you fine, it has me, but if price isnt a issue then id have gone for 100mm kingspan or celotex.
Both materials are easy to use, cut, and fix. All you will need is long enough screws and aluminum tape for all joins. The ybs superquilt place does a fixing kit for around 30 that includes a heavy-duty knife, scissors, stapler etc, and they sell the aluminium tape.
Thats how you fix it:
staple the superquilt straight to roof rafters
screw batten over the superquilt where rafters are
screw celotex/kingspan to batten
If you have separating walls id also celotex or kingspan them.