ZetaJones
Personally, i'd take it in to my own hands (but i am a bit of a control freak), possibly keeping the solicitor informed:
Check on the most recent approved plans the council has, they should be accessible on the web, what you think should be there.
Then contact the planning officer for the application to see what they think should be built and what you believe has been built.
(Ask if there is a compliance officer assigned to the development from the council too)
I'd do it by email and copy in the area sales manager and sales director for the Redrow division.
The next step would be to lodge an enforcement case, but hopefully you won't have to go that far, as hopefully they'll spring in to action and just get it done. As if they let an enforcement case get registered it should show up in the searches of anyone else trying to buy on the development (not good for sales!).
The risk is that once you complete, the enforcement action may actually be against you as the legal owner, but your contract should state Redrow will build the house to the approved plans, so you'd then have to chase them up. I'd expect the council to be reasonably pragmatic about it and just let it get built by Redrow rather than penalise you at that stage..