Bert Coules wrote:

I'm constructing a large and quite high (thirty inches or so) raised bed in
my garden for planting: block walls enclose an area of bare earth at ground
level. The bed will be topped by a layer of soil, and I've been advised to
put in a bottom layer of hardcore "to assist drainage".

You might find that during summer the block walls will cause a loss of
water by evaporation through them; and a bed raised this high might
well have plenty of natural drainage anyway. You could well have more
of a drying out too fast problem rather than a too much water one.

I suggest you ask for comments in uk.rec.gardening.

--
Terry Fields