thank you all
I think the 'repair disk' may have helped. It has also been helpful to look at Activity Monitor more closely with you guys looking over my shoulder & find out a bit more about what the various bits mean.
I am still puzzled how GraphicConverter managed to be using 1GB of real memory at the end of last night's session when it wasn't obviously doing anything. Anyway I have unticked some unecessary preferences there and in DxO as I would have to consider upgrading the box if this basic quorum of progs won't run on it.