By blacks, I meant the one coming into the box, and the one that is
part of the new switch. I optimistically, naively, (foolishly?) tried
joining them together along w the blue. Got pretty LED lights on the
switch, but nothing from the bulb.

So I've taken the advice given here and returned the thing for a far
less pretty manual spring timer similar to the one in the link provided
by Jeff Wisnia. After a small issue with the new unit not wanting to
fit into the existing box (persuaded with hammer, my favourite tool)
the whole thing works like a charm. And the wife somehow confuses my
for someone who is handy around the house, but this and a few low
voltage lights in the garden has been enough to earn a golfing pass for
tomorrow morning.

A satisfactory outcome all round. Thanks!

Dave