In reply to Longsufferingropeholder:
+1 for this.
if your loft access is 50cm wide, cut the sheets into strips 50cm wide and reassemble in the loft with some duck tape. Pretty straightforward.
There are a lot of ways to hold it in place, nothing fancy is needed as it's both light and rigid. Hard to decide what I might do myself without seeing a few photos (nails, screws, expanding foam, wedged in place, etc.)
It's a vertical wall so you need sometime rigid, anything like glass fibre of sheeps wool is a bad idea. Also insulation provided by something like celotex is a LOT more than a similar thickness of fibreglass/wool.