I guess I am trying to find out what the "official" behaviour is supposed to be. Having scoured the W3C documentation and Eric Mayer's CSS 'bible', I am none the wiser; I must be looking the wrong place! There is a shed-load of material about the rules for how floated elements float and about how floated elements behave when the containing block is too small, but there is (apparently...) a shortage of rules for how the non-floated stuff behaves in the neighbourhood of a floated element if the non-floated stuff won't fit alongside the floated element but will fit the containing block width-wise.
I thought it must be a CSS overflow property, but overflow appears only to handle the situation when an element's content is too big for the element's size.
The answer must be obvious and I am starting to feel very stupid!