text: Nice report, not just because of the detail, but basically you're saying a lot works very well. 
  
  Can't see anything specifically wrong. I've tried a route from the Watling St. end of Kelvin Drive which I think mirrors yours http://www.openstreetmap.org/directions?engine=graphhopper_foot&route=52.02049%2C-0.77099%3B52.01964%2C-0.76653#map=19/52.02017/-0.76979, and this seems to work on the website which will be with v. recent OSM data.
  
  You could try the foot routing options to see if you get different results from the app. This at least might help work out whether its the data or something odd in the app.