On Thursday, 19 December 2019 16:56:36 UTC, wrote:
Pack out the existing structure with timber running at 90 degrees to the joists. 6x3" at the high end, decreasing in depth as we move down the fall. 3" so they've got more base to sit on. This is my preferred solution, for simplicity. It boosts me up to a 1 in 20 fall. I can run the packing pieces through the table saw with a cheap blade to get them to the correct size.

I discovered the existence of Firrings. Couldn't find much online, but rang up my local Travis Perkins and they sold me 3"-0 at 4.8m for £1/metre.. And cut it in half so I could it home. They're only 1 in 64 on their own, but combined with the existing fall, that will get me to 1 in 30 when loaded, which is good enough for me.