Question
We have a small court yard garden where 'conventional' grass will not grow (due
to trees that screen back of garden and large Yew tree which dominates).  Can
you suggest living ground cover which would happily grow between slabs and
can be walked on?
I am based in Surrey, England.

Answer
Here, in the high desert of western Nevada (Where it is currently 20F and snowing!), we use the following:

Phlox (full sun)
Aron抯 Beard (part sun)
Ivy (miniature variety - part to full shade)
Potentilla neumanniana/Cinquefoil (perennial ?part shade)
Tyme (varieties range in size from 2拻 to 12拻 tall ?part shade)

It maybe not the best for walk-on, but this one is nice, none the less:
'Ingwersen's Variety' Geranium/ Geranium macrorrhizum

There are others that may work in your heavy shade condition (see link below) as an alternative to lawn that may work for you, depending on availabilty in Surrey:

http://www.urbanext.uiuc.edu/lawnchallenge/lesson2a.html

Best of luck finding them in your area!!  ~Marc