A lot would depend on which direction each of your walls faces

The short rear wall of the garage faces approximately east, and the
long side wall approximately south. I'm in Upminster, Essex, so I
guess climatic conditions are what you'd call fairly average?

Haven't actually tested my soil yet, so can't tell you anything about
it! It grows very good hydrangeas though, if that's any kind of
indication of anything - we've got two large bushes in the back garden
that were planted by the previous owner (and which we want to get rid
of when we finally attack the garden in earnest this year - hydrangeas
don't really do anything for us).

Gareth