Any update on this since september?
I ask because I'm in a similar situation - booked an mpv for Italy in august ("Zafira or similar") to accomodate 6 people. On arrival was "upgraded" to a 5 seater estate car not suitable for the pupose I'd booked it for.
CarHire3000 are not interested - say I was given a more expensive car (though this seems to depend on when you look on their website), refuse to believe that an mpv can take more than 5 people (though Europcar had no problem providing a Zafira for the same location and period and purpose 2 years ago, and I drove one at home for a couple of years recently so are well aware of it's capabilities).
I've now written them a letter suggesting legal action will follow to get some recompense for the additional costs, inconvenience, reorganisation of plans etc caused by them not delivering to contract. As with you I'm now looking to the small claims court.