Why is HS2 costing so much?

The members of the Denham Waterski Club are among the few people in the Chilterns who are grateful for HS2. They have a superb new clubhouse overlooking the tiny lake where their motorboats whizz round in ever tighter circles, thanks to the fact that their old one was in the way of the massive viaduct being built over the Colne Valley for the new line.