stuart noble wrote:

IME they rot at ground level because of prolonged contact with water,
but you can reduce the chances of that happening by having a concrete
base visible above ground and keeping the bases clear of vegetation.

I was considering resting the corners on 1/4 paving slab "pads" to deter
them from settling into the soil and shifting around, as mentioned
earlier I am considering lining them, weed membrane would be porous, so
would have to be a full waterproof membrane to stop soil within the beds
rotting them from inside out, rather than from ground-level up