RJBL wrote:

Get the drainage right, of course. Dig in a lot of limestone ballast /
limestone dust (if you can get it from your local builders merchant)
Takes several full seasons to work but the effect lasts for a very, very
long time (like decades and centuries). A cheap, more instant, fix is to
dig in alot of builder's plaster - its the same gypsum as you can buy at
10x the price at your local garden centre.

Some of it...

And I'm told, (fresh) siruphite is no longer available as something
nasty happened to the seam of gypsum.

Good luck!

Indeed.

Lots of well-rotted compost and/or manure is my prescription. The worms
will breed like good-uns, and take the stuff down and bring-up the clay,
and their holes help with drainage and aeration.

Unfortunately, worms atttact moles. Their contribution is not really
welcome.

--
Rusty