Hi, Ive been looking into icf ( still not found a suitable plot yet lol) this site has been invaluable for information for me, just seen this thread about the problems with pvc and eps.
looking at this would probably be perfect for the job.
https://www.screwfix.com/p/pipe-insulation-15-x-13mm-x-1m-64-pack/90694#product_additional_details_container