Part of the reason we are in this financial state is the push for Building Societies to demutualise. This allowed them to act in a more profligate manner.
Consider perhaps the worst offenders (or victims???) in the current crisis: Northern Rock and Halifax. Before 1997 these were mutual societies and as such could never have exposed themselves to the levels of toxic debt that has led to their downfall.
Nationwide and Yorkshire Building Societies negotiated a temporary change to the terms of the FSCS to protect members of the societies they acquired in December 2008. The amended terms allow former members of multiple societies which merge into one to maintain multiple entitlements to FSCS protection until 30 September 2009, so (for example) a member with �50,000 in each of Nationwide, Cheshire and Derbyshire would retain �150,000 of FSCS protection for their funds in the merged Nationwide.
http://www.moneymadeclear.fsa.gov.uk/news/prod uct/bs_merger_compensation.html