Your editorial “Dark and dangerous work” (May 11, page 3), calls on London Underground and Metronet to seek a negotiated settlement over a projected £750m cost overrun.

This figure was produced by the PPP arbiter when he undertook his first annual report of Metronet’s performance last year. He concluded that Metronet had not performed its activities in an economic or efficient manner, or in line with good industry practice.

London Underground is unable to confirm this £750m figure because Metronet has been unwilling to provide us with the information we need to fully understand its poor performance or the financial consequences.

Whatever the figure, it was not reached by extra coats of paint at Lancaster Gate, as Metronet suggests. More relevant is that, to date, Metronet has completed only 28 of the 56 station renewals scheduled.

PPP contracts are supposed to transfer risk away from the public sector. Simply accepting Metronet’s demands and handing over more funds is reckless and would be a dereliction of our duty.

It is important that Metronet’s performance do not distract from the ultimate objective – the renewal of London underground. All parties must continue to invest to ensure we maintain London’s prosperity and growth.

Tim O’Toole, managing director, London Underground