H Ryder wrote:

My soil is compacted clay, with hardly any worms. Have decided to try double
digging the borders. Is this a good idea? What should I dig in - I've got
some sharp sand and compost. Will this do - I can't really afford grit .
Would appreciate any advice

The easiest way is to add a mulch of compost on top. And let the worms
do the work of digging it in! Double digging may not help that much if
the deeper subsoil is (as seems likely) even purer clay.

Digging when it is not dry rock hard, wet enough to lubricate the spade
but not to stick to everything thick layers makes life easier.
Unfortunately there is a very narrow window where digging clay is easy.

Also what is the best technique for actually breaking up the great, hard
lumps. Hitting them with a spade just seems to make them harder! TIA,

Wait until winter and let the frosts get at them. With or without the
aid of a proprietory clay breaker to flocculate the clay. Loads of
organic bulky material on the top will help a lot.

Regards,
Martin Brown