Estimating the losses to UK banks from the house price crash

Here are my workings on the worst-case total house-purchase related negative equity in the UK (click to enlarge):
The total negative equity of £74 billion is around 5% of total UK household debt of £1,409 billion (which is of course much the same figure as bank lending to UK households). 

£74 billion is probably far larger than the loss that mortgage lenders will suffer. I'd expect that most home-owners who are in negative equity, but who don't fall to a Big D* will grin and bear it; conversely, amateur buy-to-let landlords who bought multiple properties at the peak and who are losing money every month are quite likely to cut and run, so the losses that mortgage lenders suffer might be half that amount, say £40 billion in round figures.
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If you want to check my workings, here are the sources:

A) The figures for loan-to-value ratios and number of borrowers in each band is taken from The Bank of England's Stability Report, April 2008, Table 1.9

B) Midpoint of range in the first Column A)

C) Per cent of mortgagors from the second Column A) multiplied by 11.8 million outstanding mortgages.

D) Mortgage value assumes that the two groups most likely to be affected by negative equity are first time buyers and buy-to-letters. The average price paid by first time buyers (which I shall take as a proxy for the average price paid by Buy-to-let landlords) at the peak of the market in 2007 was £160,000. Hence the figure in Column D) is the figure in the first column A) multiplied by £160,000.

E) The 832 respondents who responded to my thoroughly researched and highly scientific fun on line poll showed a consensus expectation of a 42% fall in house prices from peak-to-trough. The figure in Column E) is thus £160,000 x 58%.

F) Column F) is simply the mortgage value from Column D) minus property value from Column E), giving a negative equity-per-property figure.

G) Column G) is the number of borrowers from Column C) multiplied by the negative-equity-per-property from Column F)

* The Big D's are Divorce, Death, Debt, Disability, Dole, Drink, Drugs and Detainment at Her Majesty's Pleasure.