A few things to clarify:
Have a look at Google Earth - you'd easily get 6 platforms in the space from P16 (including 16 itself which was to be given over to HS2) to the other side of Cardington St, which means a whole block to develop and sell. That's a lot of money!
It's the value of the land which can be used for development. Six platforms (assuming P16 is included in this as per the original plan) will fit almost within the old station footprint, leaving a whole block to develop and sell for an absolute fortune - this is about as prime as UK building land gets.
Unfortunately the land to the West of Cardington / Melton Street is smack bang in the middle of two protected views: St Paul’s from Primrose Hill and the Palace of Westminster from Parliament St. You cannot build height there, which means it isn’t worth that much, comparitivley speaking. It also makes it ideal for low rise development above, say, a major transport interchange.
Manchester won't be a great deal slower with HS2 trains joining the Trent Valley north of Lichfield.
Just 30 minutes or so slower.
Its also got a devolved assembly to fund it if they want that.
The Welsh Government does not have delegated funding for rail infrastructure.
15 minute turnarounds and one platform in reserve. It's tight but it is potentially doable, although it will require compromises and more complex train planning.
The train planning is easy. The compromises are not, especially the reliability implications of 15 minute turnarounds when most trains are spending much of their journey on the classic network.
what will the cost be of allowing HS2 compatiable trains to run to Manchester?
nothing additional to what is already committed. All HS2 trains were already proposed to be able to use the existing network.
With so much in tunnels, what proportion of Handsacre-Euston will actually be high speed? Daft question: because they are smaller, could classic-compatibles go quicker in tunnels than captive units?
All of it. Most of the tunnels are 300km/h or more (where the trains can use it).
Like they have to do now. We're just asking Euston to cope with what it has coped with since the mid 2000s.
But we’re not. There will be more trains into Euston, specifically using the paths released by HS2 south of Rugby.
I would also have a fiver on the contract with Alstom & Hitachi to be varied to have most trains at a length around 250 metres to replicate the length of 11 car Pendolinos. But obviously with many more seats.
250m long trains would also simply Euston somewhat.