I just had a window and door replaced with a new pvc window only.
The outside brick skin was bricked up to window cill height previously (over the old door) and then we had the door and window removed and replacement fitted onto new outside brickwork. We can now block up internal wall where the door was to complete the internal cill. What is the best way to now seal the cavity which is exposed all the way round the new window as there is obviously no dpc just a void.I cannot just brick it in without fitting a dpc (awkward).
What about foam or is that just cheating?

Foam I would avoid. Use Thermabate (or similar).

http://www.thermabate.co.uk/pages/Thermabate.asp