Hmm ok... so monitoring of my brew-room puts it at between 18 and 16C depending on the time of day (I have a nifty little USB thermometer and a spare laptop to record the temperature over time with 1-minute intervals). The brew *looked* done insofar as the airlock was concerned, so I pulled the carboy out of there and sat it on the kitchen table for the diacetyl rest I usually do (just in case). Within a couple of hours the damn thing had woken right back up again.
I know gravity is the only way to really test "done-ness", but in my experience once the thing has stopped bubbling at all for three days it's finished primary without some kind of intervention (purists be damned). Since I buggered my hydrometer and am waiting on delivery of a refractometer, that was and is the only option I have.
Anyways; rambling aside - I can only assume that this yeast in fact really likes the slightly warmer temperatures in the kitchen (20C), and that it awoke again due to not being too damn cold. No other brew I've done has started bubbling away so vigorously so long (hours) after it was moved elsewhere.
Take away point: This yeast seems to like *slightly* warmer temperatures than you may normally use for ale yeasts, unless this is a fluke of circumstance.