Well, 'er indoors came home and set me a load of tasks around the house
so I've had to put off further experimentation until tomorrow. However,
the partially-blown theory looks good, because I've hooked one of the 555
boards up to a fresh 317 which is just inserted in breadboard with NO
'supporting' caps at all and it was like *totally* fine. In fact I was so
sure it wouldn't work that I didn't bother to set the 12V up first, so
ended up with 22V for about 10 seconds across the 555. AIUI, the max for
this device is 15V, so I was convinced I'd destroyed it, but amazingly it
survived! Reset the Vout to 12 and reconnected and everything was
completely normal. UNLESS of course I've done some *partial* damage to
the 555 in the same way as I appear to have done with the first 317, of
course.
Definitive results tomorrow when I'll swap out the known good 317 for the
duff one.