Given the high level of bullingdon club involvement in this latest resignation, one could be forgiven for suspecting that the copper has been used as a buffer for Dave.
Regarding corruption, anytime you expect a greater degree of morality from your public servants than from the public they serve you are setting yourself up for a fall. Therefore these things should be structurally prevented, i.e the system is self regulating. Expenses supported by receipts style, personal gratuities entirely verboten, so no argument if one is taken you are fired, sort of thing. It needs to be expected that some people will be corrupt, rather than hands be thrown up in horror when it transpires that some are.