Hi everybody, I just surfed in here.
As I'm sure you know, THE defining book of what goes where on the railway is the Sectional Appendix. Unfortunately neither Mitcham Eastfields nor St Pancras International Low Level have appeared as new inserts for the SA yet (well not my copy anyway). When I get the chance I'll have a look at the latest Periodical Operating Notice and so long as they're still in there I'll post the mileages on here.
In the meantime the best I can do is tell you what you probably already know.
1) Eastfields Level Crossing is located at 9m and 19ch. Since this lies bang slap between the up and down platforms it could possibly be defined as the position of the new station, but I'll confirm when I can. Streatham is definitely at 7m 48ch and Mitcham Jn is at 10m 30ch.
2) Kings X tunnel extends between 1m 59ch and 2m 56ch. Obviously St Pancras International Low Level is somewhere in between. Farringdon is at 0m 62ch and West Hampstead T/L is at 3m 73ch.
HTH,
Fying Pig