Andy H52
Yes tunnelling is very very expensive, especially if critical facilities are on top of it!!
The LGW plans require no public access tunnels just a few minor under tax way tunnels for ramp traffic, little more than a culvert with a top on. The A23 which currently goes under the airport to the west of the rail link will be moved in an arc back towards the M23 with all long stay car parks served from it and a new airport link to the M23 at the current site of J9.
Lets see what the Davis commission has to say, it will be either LGW or LHR not a regional airport for sure