I just came across this list of reasons why a sale is delayed though in our experience the top three are 1) banks taking forever to get a valuation and then more time to reach mortgage offer 2) solicitors moving at a snails pace and 3) management companies for leasehold properties taking forever to get back with answers to simple questions and then charging through the nose for the information.
http://estateagentnetworking.co.uk/top-10-causes-of-delays-when-moving-home/