Continuing the saga:
The charger has a battery test facility.
This shows a) total voltage 3.7V rather than the 11.1 to be expected.
This seems to explain the "no battery" message,because the charger is looking for the 11.1v to be expected from 3S.
However when the battery is metered,it shows 11.1V. This
suggests to me that the charger is not detecting two of the cells.I can only now get a new charger and hope I am correct!
Thanks for all replies.