Okay, I am stupid, I work a lot of shifts (I work with people with dementia) and yesterday's late shift was particularly hard, didn't get home until 22:45. I was trying out new icon themes in the early hours after getting home. I managed to delete /usr/share/icons whilst trying to delete an icon theme I did not like by placing an errant space in the path .
I have re-installed everything in synaptic that contains "icon", but I am missing the icons in the two columns of the right hand pane of synaptic, which makes it hard to use!
I have rebuilt the icon cache for all themes (but haven't rebooted yet).