Thanks, I really appreciate you made the effort to find some actual stats, but with respect, that doc is from march 2012. I don't know if the data is still relevant now (though it may be).

We need some solid stats by an independent party which is why I'm happy one is going to be looking into it.

FYI from that document, let me quote cos it's interesting: cost estimates in march 2012:

2.9 Route package cost summaries

2.9.1 The phase one total cost estimate is £16.3 billion within a cost range of £15.4 billion to £17.3 billion. [...]

2.9.2 The phase two total cost estimate is £17.1 billion within a cost range of £15.7 billion to £18.7 billion. [...]

2.9.3 The full Y network cost estimate is £33.4 billion within a cost range of £30.9 billion to £36.0 billion. [...]

Construction costs haven’t changed all that much since then. A significant proportion of the costs are in project management and contractual arrangements designed to shift risk away from politicians and managers. If you look at costs in France, we are about 5-10 times more expensive for a route of equivalent complexity. French digger drivers don’t earn 5x less and materials are not 5x cheaper in France, if they were French would be the language of building sites in Britain. So the costs must be coming from somewhere else; land acquisition, risk management and over complicated contracting and project management processes are my bet, based on my experience of working on large construction projects in the UK and Europe.