Looks like I came in from a winter ride and wiped bike in cursory manner not realising I had been cycling through puddles with salt in them.
Thus chain seized and despite GT85 and WD40 sprayings, still four links stuck. Have put in bath of engine oil. Shall leave for period of time.
Back wheel has little white circular stains. I tried pan scourer gingerly, this was also wrong. But did it very gently. Now contemplating nail varnish remover.