C# is on p3 of this 2015 map.
If I'm reading it rigt it shows the separation as ~25m but not the depth.

Greater London Boroughs, Slough and Buckinghamshire
Ref: Volume 1PDF, 109MB, 21 pages

link to .gov page :
https://www.gov.uk/government/publications/hs2-property-schemes-maps-greater-london 

I'd be nervous about excavations so close to my house too.
It'd be interesting to see a cross section.
is it worth an email to them for some more information & reassurance?
There's an address on that page.