A quick update ...
I've been putting the partly-drawn mating nuc frames (especially those with the wide all-round starter strips) into mating nuc boxes above a strong queenright colony - in order that they can be held somewhere while I get the remainder drawn-out (in pairs), and at the same time this provides an opportunity for any existing brood on those combs to emerge.
Being above a strong colony, the temperature in those mating-nuc boxes will be higher than in the single-story hives in which those combs were initially drawn-out, and so this is typical of what has now resulted:
As you can see, the side adhesions are much improved and there's also now attachment at the bottom - and so I'm now even more convinced that temperature is playing a key role here.
However, this also suggests that there are competing dynamics present: in order to get foundationless combs well drawn-out, it's desirable to have a reasonably high temperature within the immediate comb drawing area - BUT - the only way such a temperature is guaranteed (bearing in mind we're talking Britain, not Florida) is above a strong colony, and at this time of the year a strong colony will draw drone comb instead of worker comb.
So - we need a small colony to ensure worker comb is drawn, yet a big colony to provide the heat for this ... Hah.
It's looking like the next step will be to house a relatively small nucleus colony in a nuc stack and supply it with artificial heat such that the temperature in the top box (where comb-drawing will take place) will be up in the 90's Fahrenheit.
Will keep you posted.
LJ