A levy on properties worth over £1 million

Some questions and comments spring to mind about this Lib Dem policy:
How will property values be assessed? At the last rating valuation, at the top of the market, at last purchase, at estate agents valuation (if so which agent), at average for the road, at highest value in the area, at lowest value in the area? Not as easy as it sounds is it oh economic genius Vince Cable?

This policy will unduly penalise Londoners over the rest of the Country. Whilst a nice house almost anywhere in the Country can be purchased for less than £1 million, in London it is not quite that clear-cut; but then a bit of class war rhetoric by the Lib Dems may help shore up their vote against a Labour party also trying to shore up its core vote.

Vince Cable appears to be assuming that everyone that owns an expensive house has high income, what about retired people in large old family homes, say around Richmond Green? Will they be forced to sell their houses as they may not be able to afford the 0.5% "levy"?

The levy is set to start at 0.5%, £5,000 on a £1 million house, will there be a set upper limit on the rate of the levy or will it like stamp duty be hiked ever upwards as an extra "wealth" tax?