One has packed up, where the soft-close has stopped working for the seat, but is still working for the cover.
Finally got round to removing it for inspection, today, and was a little perplexed by the construction.
Looking at the underside, there are 2 small threads with 7mm nylox nuts that emerge into the centre of the assembly from the outside. However how the seat/cover were actually put in place seems a mystery, as there's no movement left-right to allow it.
I was able to remove the nuts (so there's something holding the shaft) but beyond that can't see how to proceed.
It's tempting to abandon the thing and just get a new one, but SWMBO has remembered that we've already done that - which is the one I am now looking to repair.