Dear Val,
The GCR route includes the portion of the joint GCR/GWR line from Ashendon Junction. I have tried to model the section southwards to just south of Princes Risborough (see GCR V15) and intend to go to High Wycombe and beyond at some time in the future!!!! I do hope that you are able to make good progress with your overall project and have lots of fun.
Regards,
Peter