> Eventually solved it by tieing the (floating) programming pins
> high. I'd not seen it before. I know you're supposed to "do
> something" with all the "unused" io pins, but hey, I'm a hobbyist
> and I've got away without doing it before

The easiest "something" is to set them to outputs. If there's a
chance a track could be shorted to the opposite polarity, add
a series resistor, one that will limit the pin current (20 - 25mA).
Pins that need to be inputs but at some time may be uncommitted,
for example when an input device is removed/unplugged, should
have a pull-up or pull-down resistor so they don't float

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