A-Level? I'll have you know I have a 15 year old Desmond in 'Economics and getting too drunk to bother with lectures.'
The balance of probabilities isn't really the point - what I'm taking issue with is the idea that new build properties will necessarily bring prices down, I'm not arguing they definitely won't. THe problem as I see it with applying your basic supply/demand logic to housing is that of defining what's being traded and what 'the market' you're looking at is. As far as the product goes, all houses/flats are different. A flat near a station or a good school can be far more expensive than an identical one a few hundred metres away. In terms of the market - you could look at the market for Brixton housing, London housing, housing in the south East, or even housing on a particular road, and see different effects. It's that geographic element that the basic market model doesn't account for (and is beyond my knowledge tbh.)
Personally I think the effect you and others have been talking about is probably right London wide. The massive rise in house prices across the city has been driven by lack of supply and more houses will push the price down. So to an extent Brixton prices will be pushed down. Against that though building a load of new build, and the resulting facilities that are set up to supply the new residents, could easily create an increase in more localised demand that might outweigh that reduction altogether.
BTW I'm not saying they shouldn't be built - I absolutely agree more housing is needed and that that includes private housing as well as social housing. And if you build enough London wide that would reduce prices for everyone, or at least stabilise the rises. I just don't agree that a new build will automatically reduce prices in the immediate area.