Adrian C wrote:

Also, check the 7812 has a couple of nearby 0.1 microfarad caps on both
input and output legs of the device. They are known for oscillating
without, and drawing excessive current in this fault mode.

Yep, did that. Couldn't swear to the actual values as they were out of
my parts bin, but they would have been reasonably close to what was
recommended and I guess the value's not too critical.

I've fully subscribed to the fan-supply-overloaded hypothesis and am now
just looking for a suitable small supply for the electronics only. Most
packaged supplies I can find seem to be too big, and therefore
presumably inefficient at the required load.

Pete