I finally unstuck a very stuck pedal. It had resisted:
Pedal spanner;
penetrating oil and then the pedal spanner;
the big bloke in the bikeshop and his pedal spanner;
"Cheater bar" fashioned from the leg of the bike stand;
Some digging around online suggested applying heat to the crank (alloy expands more than steel) but I don't have a heat gun so resorted to boiling the kettle and putting the pedal and crankarm in it. Left it for 10 minutes and then finally managed to get the pedal off
so it's time for a
...I'd better wash the kettle out first