not being completely convinced, I popped the laptop in the car with a note to drop into the first "Computer repair" shop I passed (there seem to be a few around).
Sure enough, popping out this afternoon, I passed a place, so dropped in.
Guy found the correct charger (which looked identical to the one I was trying), plugged it in. Bingo !!! Powered up and made a ping. £20 for the charger. Annoyingly I was convinced I had a spare AC lead with the 3- circles, but if I have, it's buried. I've borrowed one from another laptop for now.
Looking at the new charger and one which didn't work it seems the central connector is slightly smaller.
Oh well - I was wrong about my charger. But that's experience. Having been stung by things that should be identical on paper, it was worth the 5 minutes to pop in.
(I know I could have got a charger for £10, but AFAIAC the guy in the shop earned his £20 ....)