An update and some thoughts on my new friend's behaviour...
Being close to the fixed reference surface of the underside of a cupboard I noticed that the leaves grew at night and shrank in the day; apparently this is normal for plants due to reduces transpiration from the leaves at night; causing pressure inside the leaves to increase along with their volume.
The second thing is the orientation / appearance of the leaves. The leaves generally grew pretty vertically, but when the larger ones started to impinge on the underside of the cupboard I moved the plant back to the table, where it gets more light. Over a day or two the leaves started to level out and "droop" more.
This, along with them maybe looking a little limp caused concern for its welfare so I moved it back into the shade and the leaves are now becoming more vertical again - perhaps a mechanism to seek out light by growing beyond whatever is shading the plant..?
The hydration situation has also changed recently. Previously the pot was inside another of maybe half its height, and by the end I'd been pretty blase about just filling the outer pot to the top with water... which the plant apparently yummed up. I've since changed to a much shallower dish to stand the pot in, which gets topped up with water but obviously provides less as there's less depth of soil actually immersed in water.. The plant was maybe looking a bit limp the other day but given its behaviour upon being moved I think the limpness was due to light availability rather than water.
That is all
EDIT: Just been reading
this, which piles on the importance of watering correctly and also suggests that they don't like hard water - which is ironic since it's proper Chuck Norris round here. Maybe some of the Chlorine has at least evaporated out as I tend to use old water (perhaps some that's sat in a glass overnight) for the plants..