...a large trench constructed in the floor of the existing tunnel and stretching the full distance from the centre to the outside will produce, indisputably, a larger tunnel.
Aye, and a tunnel needs an entrance. I can see it now, the Atkinson/BBC door and lintel finally and properly removed, but in its place a small state-of-the-art door bearing the year 2008, and beneath it a squashed millennium bug highlighted in red cloisonné.Silbury no longer needs the 'sound of iron on stone...'