It is known as "feeding the beast'' at some top law firms - the much-loathed task of working out how much time can be billed out to clients at the end of a long working day.

From old-school paper time sheets scribbled out by hand to sophisticated software programmes that allow lawyers to capture each and every minute they spend at their desks, time remains the prime currency of Britain's £20bn legal services market. Walk into any young solicitor's office and you are likely to glimpse a series of timers flickering on his or her computer screen - tiny icons that can be clicked "on" and "off" each time an e-mail is drafted, a document proofed or a conference call convened.