In the event that they do require the employee to pay it all back, what I would have done was try to make the original wage work on the budget and stash the other £18k into low risk index funds to get some dividends and increased return. That way if paying it back were necessary, you'd at least make some money from the excess capital.
Thats the part where an employee should do what employers do all day every day like breathing: lie your ass off.
"I recall the verbal agreement being for 42k, just as it is in the contract. Stop lying."
Probably should look for other work once the error found out though regardless of how you handle it. They'll invent a reason to fire you for failing to be a good lil wage slave, cowering and signing an hr document obligating you to pay the difference back. A month later it will be some "not a team player" dismissal.
Fortunately though, it might never be discovered. Corporate malice is matched only by corporate incompetence.