Its still running, and the old design is holding-up .  I found a circuit diagram in the archives and realise there is yet another bit of bodgery that needs to be sorted if this is to be used for long duration unattended transmissions.

The 12V supply used for the fan and other bits of control circuitry is derived from the input supply by a single LM317 regulator.  That device is specified for 40V maximum across it, so at 50V input gives just a 2V margin.   Nowadays, when I want a regulator from 50V to something low, I use an auxiliary emitter follower to coarsely pre-regulate the input to the main regulator to some lower intermediate and safe value.

Will inform here when it finally goes off tonight

'jnt