Thanks for that Rod.
Alas Ive No idea whether it's the old encoder or a new one that was fitted with the box (about four years ago). It's operation is completely unpredictable and it still stops squawking, even after it has been operating normally for quite some time. Frequently it seems to go on the blink with a change if squawk code, but sometimes it just goes on and off in a random fashion. All indications on the unit itself are entirely normal inc flight level readout. When it is working properly cross checking with ATC shows correct altitude and normal returns!