Mac the Knife,
Thanks for your advice. I had tried cleaning the drive with a head cleaner some months ago but it had no effect. I had thought of trying to format the dead floppies on a Macintosh but have not been able to find one. In three years here I have only heard of one Mac in the whole country, an eMac that a teacher brought in last year.
Last Friday I obtained a copy of Formatt144 and ran ten dead floppies through it. Four formatted first go. Two more were formatted at the second attempt and another two after several efforts. I am very pleased with this. It’s too bad I threw out some of the floppies before I realised the extent of the problem. Anyway, it seems I can keep the existing stock of floppies in action for some time, so that has solved our immediate problem.
Thanks to all who have contributed advice and information.