Having worked in a garage, and I can assure you that anyone else who has will also tell you this, you're buying a seven year old car with known faults.
As Postdog has noted, you were well aware of the potential fault (note I say potential as it will not necessarily affect all models), yet you still bought it - case of buyer beware?
Have you established for certain that the water is coming in from the door seals? The amount of water leaks I used to investigate on customers cars would inevitably be coming in from somewhere other than we thought they would.
Water in the foot wells could be indicative fo a leaking heater matrix (common on some Renaults), a leaking sunroof, blocked drainage in the doors, and I could go on.
What car is it exactly? I'd be very wary of believing everything that Parkers tells you about a car - it's not got a good name in the trade and insurers refuse to use it.
Back to the matter at hand though, you should have a three month warranty (minimum) in which the vendor is obliged to repair the faults (assuming it's not caused by normal wear and tear, or paint chips on a 200000 miles plus car), so there shouldn't be an issue there.
Obviously if the fault has occurred after the three months is up, you've got very little comeback there - I bought a convertible that was great in the light rain, but once the downpours started, it was like a swimming pool. I knew this particular model was prone to leaking, so what could I do?
And any extended warranty is highly unlikely to cover water leaks of this nature