Thanks to everyone for the replies. As usual with this sort of area, the
range of advice is extensive.

I have a source of old concrete panels which I can break up and use for
hardcore, but I'll have to bring in some smaller aggregate to use as a
filler material and to level off the top of the base layer - without that, I
suspect that any membrane, even the toughest, would be penetrated by the
edges of the concrete shards.

I've also been told that broken-up chunks of expanded polystyrene made a
good substitute for traditional hardcore (and would certainly be easier to
handle) but, as with something harder, I'm still not sure I completely
understand the way the drainage is supposed to work. However, any
moderately sizeable material would at least cut down on the amount of
smaller infill necessary, which I suppose would be a saving.

Newshound, particular thanks for the link to the Paving Expert site: what a
splendid resource.

Bert