Oops - think I've just cooked my heater
#1
Was in the middle of a larger than usual water change and was leaning over the tank to put some new plants in ... what's that strange smell ... hmmm, there's smoke coming from the heater, unusual ... aaarrrrggghhhh

Had taken the water below the level of the heater thus heater had gone into overdrive and was gently cooking. Could have sworn I'd turned the thing off in the summer, apparently not.
I'm guessing that's the end of it then, can't imagine it will be reliable now having got to that heat, it sizzled as I took it out of the holder and was too hot to hold through several folds of towel.
So let that be a warning to everyone else! Always turn off heaters (or, if you think they are turned off, check that they actually are

) if the water level is going to go beneath them when you do a water change
It's Not Just A Fish