To recapitalise a bank, the bank needs to receive new money by issuing new shares. This has the effect of diluting the existing shareholders and reducing the value of the existing shares as future profit now have to be shared out to more shareholders. The usual way to do this is to issue new ordinary shares but in the UK, the banks are issuing preferential shares which have a prior claim on the company to ordinary shareholders.
I think you can safely say that the market has already assumed that this is going to happen and have marked the shares down as low as they currently stand.
If new shares, and particularly preference shares, are issued in a great quantity then it could be argued that even at the current low prices the existing shares are overvalued.
A lot depends on the state of the banks loan books and the information in the public domain at the moment is not very complete nor open with particular regard to the loans to developers.
Without detailed and verified information, it is impossible for an investor to evaluate the share price. Hopefully, the on-going investigation will shed some light on this, or else we will have to wait for the 2008 results due next Spring.