I have just started reading Laura Laker's book "Potholes and pavements" which is a trip round the National Cycle Network.
In the back there is a reference to "Study of disused railways in England and Wales - Potential cycle routes" published by HMSO in 1982. Produced by John Grimshaw for the Department of Transport. A copy can be found at
https://files.websitebuilder.prositehosting.co.uk/4e/a6/4ea60cd3-c959-422f-a256-ce418dc3238f.pdf which only appears to be a part copy.
It obviously relates to a very different world to now, but there are some interesting facts and graphs before Excel (or maybe Supercalc). The Bitton-Bath & Ashton-Pill Bike Path is used as a case study.
Looks like I have two interesting books to read.
Unfortunately I have not found the map that is supposed to be part of the report.