Didn't spot that bit about my thread.
Round the back it's just a couple of layers of Knauf loft insulation. One layer round the cylinder, and another bit to fill in the corner. Then as finishing off the bits that aren't hidden, I slipped some into bin liners to make it less messy and packed some more spaces.
Boiler's been fine since I did it, and the tenants seem happy enough with it. Happy tenants pay the rent, that makes me happy

Edit: I keep meaning to see if I can borrow an infra red camera and see what it's like for heat leakage now it's finished.