A bit of research showed that my charger was faulty but it was acting as if the batteries were faulty!
There are two small fuses internally winthin the charger. I did a buzz test on both & found one of them to be blown. I soldered in a 5A car fuse (to replace the 5A fuse that was there). Hey presto, it all works now & Ihave three working batteries!
Thanks for the replies.