"2987fr" wrote:

It could be a mechanical problem if one of the feet is broken internally
or you now have a cracked trace to one of the load cells.

Spot on. When I opened it up I found that one of the two rear feet which
act as sensors (each connected to the main PCB by a single wire) has come
adrift from its fixings (very small plastic studs on the case, seemingly
just push-fitted into holes on the internal flange of the foot.

I'll try refixing the flange (Araldite, maybe?) and see if that solves the
problem. In the meantime, many thanks for all the thoughts and suggestions.