For a journey from Plymouth Group to Reading, you can use maps SW, SW+WR, WE, WR, WR+SW, or ZZ.
That is Bodmin - Plymouth, then
Plymouth-Taunton
Taunton - Westbury - Reading
OR
Taunton - Bristol Temple Meads - Bath Spa - Swindon - Reading
OR
Taunton - Bristol Parkway - Swindon - Reading
OR
Taunton - Westbury - Bath Spa - Swindon - Reading
OR
Taunton - Westbury - Bath Spa - Bristol Temple Meads - Bristol Parkway - Swindon - Reading
All using maps ZZ, SW, WE or WR.
SW + WR and WR + SW do not give any extra options.
In particular, Bristol - Cheltenham Spa, Cheltenham Spa - Swindon - Reading doubles back between Standish Junction and Cheltenham Spa, including the entire section Gloucester-Cheltenham Spa.
This is shown clearly on Google Maps,
http://maps.google.co.uk/maps?q=sto...use,+Gloucestershire,+United+Kingdom&t=m&z=14 but not so clearly on the routeing guide.