Tourists today might note a definite tilt in the clock tower if they look closely but Burland says it is not caused by the London Underground; it has always been there.

"It's been known for more than a hundred years. It's probably been leaning since it was built in 1858."

Burland bases his argument on the condition of the stone cladding: "If the tower had started to lean to the left after the cladding was applied, the cladding would have been very badly damaged. There is no damage there, which leads us to believe it must have leant pretty early on, while they were still putting the cladding on." He estimates the tower's lean at a gradient of about one in 250.

"It's just about visible. Much more than that and people get a little bit uncomfortable. But it takes quite a lot more to need remediation-one in 100." The Pisa tower, he says, had a lean of one in ten. "It's terrifying if you walk on the side of Pisa, especially when clouds are moving."

At the time, compensation grouting used to prevent further leaning was a relatively new technique. Today new, automated methods of tunneling create much less ground movement.

"If we were building [the extension] now, we would have done it differently," Burland says. The high-speed rail link to the Channel Tunnel, which opened in 2003, "was constructed under very sensitive buildings and used one of the much more modern tunneling machines, and the movements were much smaller. There was a lot learned on the Jubilee Line Extension."

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