On Fri, 31 Jul 2020 00:20:32 -0700 (PDT), Tricky Dicky
wrote:

. I am looking at fixing 25 X 25 battens to the bottom of the joists and dropping 75mm foam insulation in between the joists. Any advice regards preferences
between Kingspan or Celutex or is it just a case of whichever I can get at the best price?


Either, or similar such as Xtratherm or Ecotherm. See
https://www.secondsandco.co.uk/ for a cheap supplier as long as you
are using quite a lot. Small amounts only make sense if collecting
from Presteigne in Powys or Stirling.