On Sat, 08 Aug 2009 10:57:33 +0100 (BST), "Dave Liquorice"
wrote:

I'd remove the rubble, there is small risk that it could form a damp
bridge. If you have some suitable insulation put it in the gap. Think
about damp proof courses and not bridging the cavity when you
concrete over.

That's what I was thinking, thanks for confirming that. I don't have
anything suitable to hand, so what do you suggest: a small sheet of
polystyrene from Wickes? Thanks again.