A further definition from The (Alternative) Legal Costs Dictionary:
Detailed assessment n. a process whereby a receiving party seeks to recover the cost of work that was not actually performed and the cost of work that was performed but for which there was no need. In turn, the paying party seeks to argue that the case should have been conducted in half the time, at quarter the cost and by the office cat, if at all. The process is usually overseen by a court official with no interest in the outcome and no understanding of the rules. An entirely arbitrary process elevated to a cornerstone of the English legal system.