My heart was in my mouth, I could barely look.... but actually, it was remarkably easy, and now my one monster triffid is three baby triffids.
I've hedged my bets with three possible overwintering options:
At DEFCON 1 is Triffid 1:
Most at risk of being lost to frost damage, especially if we have another winter like the last one, Triffid 1 is outside, in the border, and will stay outside all winter. I'll be giving it a thick mulch of autumn leaves, topped off with a pinned-down plastic bag (to keep the water off the crown as much as possible) covered in compost for extra insulation.
At DEFCON 2 is Triffid 2:
(can you tell this is the knobbly bit that was trying to get out of the original pot?)
Potted up into a nice 50:50 mix of John Innes and multi-purpose, plus a handful of sand thrown in for drainage, this chap is going into the frost-free greenhouse to see out the winter. I'm not too confident, to be honest, as I did this to my majestically lush Ensete ventricosum 'Maurelii' last year, and that's supposed to put up with roughly the same conditions as a Hedychium - but it turned to mush pretty quickly.
And at DEFCON 5 is my fall-back position, the one I really ought to be able to get to survive, the one I'm risking domestic peace and tranquillity to preserve by putting it in the dining room for the winter:
Actually there's a sting in the tail, as this was the only chunk which came away without a big hunk of root on it. It had some very sturdy-looking side roots which I hope are even as I write developing into the large rope-like snakes of its larger brothers. But in the meantime I've had to support it with a network of canes to keep it upright and the roots, such as they are, as stable as possible.
I chopped them all back by about two-thirds so the root systems wouldn't have the bother of supporting nine-foot greenery as well as finding their way around their containers (or my other plants, in the case of Triffid 1). Now I just have to watch every single weather forecast for that crucial first frost: in fact I think even if one hasn't arrived by next week or so I'm bringing them in anyway. Wish me luck.