A spit deep with clay underneath is the normal situation here in my part of London - if its not a pale, yellowy colour, but more dark orange, then it's okay to break it up a bit - if it is a pale yellowy colour, that means its subsoil and shouldn't be brought to the top. I suggest you incorporate as much organic matter as you can get hold of into the top layer - well composted horse manure, leaf mould, organic soil conditioning compost from the garden centre, home made garden compost, anything like that. Double digging to break up the clay and work it through the rest of the soil you have would be no bad thing, but is very hard work, and only good if you're sure its not subsoil you're breaking up. When you plant, dig a hole down to the clay and break up the bottom of the hole with a garden fork, backfilling with the soil from the hole to actually plant.

Some fine grit mixed with the organics wouldn't come amiss either.

Thanks for that, I will give that a try.