"This used to be on a better price of around £6 per canister. I used this in the loft conversion I did and it seals the celotex insulation perfectly - doesn't expand too much and lasts for a long time.
I bought quite a few and always keep one on the shelf
Screwfix is the best 'local' price and normally in stock"
"The best window / door install foam we've found. Sets to a really dense, structural type foam, not 90% air like other foams. Squirtable with good precision, you can fill gaps almost exactly, no overspill and trimming required. This is not the foam for bodging a gap somewhere, this is the foam for filling an irregular 5mm gap around a frame perfectly with no mess. Takes about 45 min to set hard enough to seal over."