Very nice idea. To be honest, I doubt you're ever going to get high-quality timely data on available places, but I think the site is still very useful if only as a guide to what property is like in the area and which estate agents to call.
A few issues/suggestions based on 5mins clicking around:
* Lose the 'label' field on each area the user picks - they're already choosing a search term for google maps, why can't you just use that?
* Having chosen two places and chosen some property parameters, the right-hand-side of the page contains two 'property' buttons: one grey, which does nothing, and one orange, which allows me to reset the parameters. The grey one is useless and confusing, lose it
* Having chosen these places and parameters (I chose 'crouch end' and 'dulwich'), I can't see any properties, just the standard gmaps lollipop icons on the map. I guess this is a bug? In any event it's unclear what I should do next - you'd lose me at this point
* There's a big transparent grey footer that doesn't contain anything - what's the purpose of that when you could just show more map?
HTH etc. Nice idea, will check back once in a while...