The Big Dig

Two Kerrymen went to Harold Wilson and offered to dig the Channel Tunnel for £500.
'How do you plan to do it?' asked Wilson.
'Well, said one Kerryman, 'to speed up operations, I'll start to dig on the French side, while my colleague here will start to dig on the English side'.
'But', protested Wilson, What happens if there is even a slight error in your calculations, and you fail to meet?'
Then', said the Kerryman, 'you get two Channel Tunnels for the price of one'.