It wasn't the battery box, sadly. One of the 4060s has an odd fault - it starts up fine and then the signal slowly fades away over ten or fifteen seconds, until it's totally gone. I built a simple battery box at home following a test with Andrew's system - very nice chap, by the way, and the man to go to if you want to buy a Leica camera in London -and it's obviously the mic, not the box, which is a pity. Anyone got a spare 4060 they want to sell?
Thanks for the information and offers and especially to SClassical for putting me in touch with Andrew.
Regards,
John