Each element in the report has an 'Evaluation Time'. The catch, as I'm beginning to realise, is that only those rendered 'Now' actually have any ability to expand or collapse the bands within the report. i.e. it makes an initial pass (evaluation time 'Now') and figures out the sizes, and then goes back and recalculates the content (but not the size) of the stuff.
Which is great, except that I'm trying to print something (which should disappear when empty) from a subreport, which only works if it's deferred. Guess I'll have to accept that that band can't be collapsed when empty. It's OK, I'll just redesign my report then.

/rant over.