Well my sickly Drosophyllum was chopped up today to see why it had died.
Although the base seemed reasonably ok, with both old black roots and a few newer white ones, there was extensive soft rot in the bulk of the woody stem with just a few green leaves at the top. These were fading fast with no chance of recovery. What would normally cause stem rot as I would have expected to have found it at the base of the stem if it had been caused by too much watering?

I had stood it in a water tray last summer where it grew really well, but kept it drier in the winter. Till the start of March it had looked unchanged and fine. But them the new leaves started to go limp and lose their dew and I guessed its days was numbered. A second slightly smaller plant growing alongside in identical conditions looks fine, and with the improving weather will hopefully stay that way. Still would like to know what happened to the other though...