So my conversion is complete. SLX mech and shifter, SRAM PC-1130 chain and Snail 34T chain ring.
The used XT 40-11T cassette fitted straight on with zero hassle.
It's smooth as you like, seems like it has enough range to allow for most situations although I'll probably go for a 42-11T Sunrace cassette (they look like they have larger alloy spiders to save weight) next spring when my part worn 40-11T will need replacing. No chain drops last night even without the clutch engaged but weird to only have the one shifter, felt like my first geared bike that was 6 speed!
Seems to suit the purpose of this bike.
Carl.