Well, those wretched chargers are crafty and take a bit of fooling....
I put another good 6V battery across the dud in parallel (no flow of wrigglies as flat battery appeared virtually open-ciruit to the good battery), then after 4 minutes connected charger. Light comes on - so after 2 minutes I disconnected good battery from jump leads. Light stayed on - hahaha I thought.
After another 4 minutes I had to momentarily disconnect charger so I could remove the insulated jump lead croc clips, plugged charger back in and no light .
So - put on jump leads differently so I could later remove them yet keep charger croc clips comnnected, and put good battery on line, charger worked, immediately removed good battery and jump leads with charger still connected and all good so far.
I have concluded that my intelligence, whilst not great, appears to be marginally greater than that of the charger. :cheesy: