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for moisture robustness and air tightness you need a vapour-closed membrane above (warm side) the insulation, which is air sealed to the wall. The air seal would be achieved with a thin lime 'parge' layer of mortar on the wall onto which the membrane can be taped.
For super extra efficacy, you would also put a wind proof, vapour open membrane below the insulation to prevent 'wind wash'. This is totally optional.
For durability, the insulation will be held up with either straps or by holding up the lower membrane with wood battens nailed into the joists. Stapling the lower membrane is less robust but it works ok.
would make sure you've got adequate ventilation under there - plenty of air bricks, none of them blocked up etc
gillies
Under-floorboard insulation - have i got the right idea?
bottom layer (closest to bare earth): breathable membrane (something like Tyvek?)
middle layer: the main insulation (not necessarily important whether rigid kingspan or soft rockwool type stuff)
top layer: airtight/non-breathable membrane (not sure what - could plastic sheeting suffice if gaps left by airbricks?)
essentially, I'm hoping to try to re-create this process but on the cheap if possible.