In article , tina
writes

I'm also going to buy a couple of 8ft by 4ft plaster boards & chuck
those in as well in the hope that the gypsum in the boards may help to
break the solid clay up a bit.


I would have thought gravel would be the best bet to dig in. This will
break up the clay somewhat.
--
Janet Tweedy
Dalmatian Telegraph
http://www.lancedal.demon.co.uk