I would like to know why it is a permitted route to travel (according to the NRE Journey Planner) from Wymondham to Cambridge whilst double-backing via Norwich on the way? Does anyone know if a break of journey is permitted whilst doing this on an off peak ticket or not?
Norwich is an appropriate origin routeing point (ORP) for this journey, and Cambridge is the destination routeing point (DRP).
Wymondham to Norwich is by direct train and therefore the 'local journey' to the ORP is valid.
Map KE covers the route via Wymondham; there is no doubling back on this map.
You have not stated the ticket type, but as mentioned in your other thread, the type of ticket is what determines whether or not break of journey is permitted for this journey. If the ticket type allows break of journey then yes it is valid.
Note that the above principle applies to the routeing, but this does not mean you can double call at your origin, so be sure to take a train that does not call at Wymondham.
On the return journey, if you are on a train that calls at Wymondham, the ticket ceases to have further validity when the train calls and the contract is fulfilled.