What's £40 billion between friends?

The much vaunted taxpayer-funded bank bail-out is supposedly going to cost £37 billion*. My own worst-case calculations indicate that the hit to banks from the house price crash might be in that order, call it £40 billion. This all sounds pretty grim, as does the thought that our banks are now under control of the gummint or the thought that this equates to a 1% hike in income tax for the next few decades.

So let's look at that £40 billion figure from the perspective of UK banks (workings below).

£40 billion is barely 1% of total bank sterling assets***
£40 billion is a 3.3% write-down on mortgage advances
£40 billion is three-and-a-half months' worth of gross cash receipts from existing mortgage borrowers.
£40 billion is seven-and-a-quarter months' worth of pre-tax, pre-bonus profits.
£40 billion is 1.7% of total bank bonds, mortgage backed securities (should you agree with me that the best way to solve the 'banking crisis' is debt-for-equity-swaps).

So, er, to cut a long story, why is the government telling us that banks are close to collapse? Why are they asking the taxpayer to bail them out? Seriously, why? More to the point, why are we falling for it?
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Workings:

The following figures are from the Bank of England's "Monetary & Financial Statistics (Bankstats), September 2008":

Total lending secured on dwellings: £1,216 billion (Table A5.3)
Hence, total gross interest income at typical SVR 6.5%: £79 billion
Regular repayments and other lump sum repayments on mortgages (excl. repayments on redemption) in the year to August 2008: £57 bn (Table A5.5).
Hence, total annual cash inflows from existing mortgage lending: £136 billion.
Total sterling* assets: £3,029 billion; total sterling liabilities £2,991 billion, of which £603 billion is common-or-garden current, deposit and savings accounts, so the balance is thus £2,388 billion in bonds, mortgage backed securities etc.

Further, we know that UK banks paid about £15 billion in corporation tax last year, as well as £16 billion in bonuses, so pre-tax, pre-bonuses, their profits were in the order of £66 billion.

* Follow the link for a fairly well researched article, by Torygraph standards, which outlines what this means in extra tax burden.

** There's also a shedload of foreign currency stuff, which I have ignored for now.

*** Probably a bit more than 1% if you exclude double counting, but hey.