Hi Mark,

the website for the uk.rec.gardening has advice on improving soil which I
have just tried out... using building grit (£29 per ton from wickes) to
improve drainage and plaster (finishing plaster at wickes £3 for £25), which
contains gypsum and apparently floculates the clay. Step by step procedure
at http://www.tmac.clara.co.uk/urgring/faqsoil1.htm

looks a bit weird when the soil goes white/grey from the plaster, but that
improves once you mix it in.

good luck

kind regards
Bruce