What does capping HS2 spend mean?

There's a peculiar paradox with HS2 - while the proposed costs just keep on taking from the public purse, for a blogger the whole saga just keeps giving.

I first highlighted problems in the approach away back in January and there's been a steady stream coming down the track since then. We also had the high profile reporting of the Institute of Economic Affairs expectation that the real cost is likely to be £80bn, even though Steve Ashcroft had blogged to that effect early in July. This week there appears to have been confusion among key stakeholders what the budget comprises: the Transport Secretary says £42.6bn, the Chief Executive charged with the implementation says there's an extra £7.5bn for rolling stock, and the Treasury are reported to be working on a figure of £70bn.

Now we have learnt, amid nervousness in the Labour ranks, the Shadow Transport Secretary wants to cap the spend at £50bn. What does that mean and what are the implications?

1. There needs to be clarity what the budget is supposed to cover. While there is clearly confusion already regarding the budget, if Labour propose to cap at £50bn, then what is counted within the £50bn needs to be made clear, otherwise if they come into government, they risk being castigated when the first pound over £50bn is paid.
2. What is the agreed categorisation and split between capital/revenue costs? If that isn't clear there is a danger of reallocation to mask overspend.
3. Are the consequential costs of expert opinions, compensation to those adversely impacted, the costs of legal challenges, etc., also to be met from the £50bn allocation?
4. What are the likely consequences of spending £50bn and then saying 'no more'? Will £50bn be viewed as sunk/wasted?
5. If the budget is to be capped at £50bn, what is the most appropriate way to sequence the investment to ensure, if the plug is pulled, that the maximum benefit can be achieved from the £50bn spent and how will that scrutiny be provided?

It is quite easy to make a statement of capping expenditure but I fear this could come back to haunt the Shadow Transport Secretary.

My own personal prediction is that we have not heard the last of HS2 procurement, by a long shot.