hx711 values non-linear
Having revisited this problem, I now have a solution, which is embarrassingly not an electronic one!

I wasn't mounting the load cells correctly.I had naively positioned them between 2 boards - however this meant that the central part wasn't able to flex properly against the outer ring.

Mounting the cell using the outer ring of metal only, and leaving the centre free-floating and the output became linear and scales consistently as expected.