Wow I can't believe this is still going with quite such fever.
I think most people expect there to be some sort of correction in the London/SE market. By how much, no one knows. Maybe 20%. Maybe 50%, maybe more (hope not).
I think mostpeople agree that London prices have become detached from the market fundamentals.
If the correction will be a minor blip or a wholesale crash that prices will never recover from... uncertain.
When exactly this will happen... uncertain.
IMO (and the opinion of most reasonable commentators) the absolute best thing for most people would be a very long period of stable house prices whereby house prices stay (no nominal loss but a reduction once you take inflation into account). Inflation and earnings increase affordability for the next generation, but homeowners don't get spooked into stopping all spending and staying put. No negative equity issues so people can still move and keep liquidity in the market.
The property industry likes to call stable house prices 'stagnation' and make it sound like a bad thing. But it isn't.
Massive drops won't happen in isolation from the rest of the economy - so a 'crash' will lead to all round bad times. If property values halve, people will be left with big negative equity and repossessions will go up, banks balance sheets suddenly look very shaky and there is a constriction in lending (not just against homes, but business lending too). Add to that massively reduced consumer confidence and spending, recession, loss of jobs etc.