I occasionally buy a First Off Peak Return Westbury - Newbury. On the return Journey I ask the
TM‡ for an excess to Frome which is Zero. I've had this done a number of times without issue. Some TMs however don't like issuing Zero Excess tickets as it can cause someone in the hierarchy to get upset (Or so I'm told). Usually however it is issued.
On the occasions it hasn't been I've changed trains at Westbury to Frome and explained the problem to the Guard on the Frome train. Showing a screenshot of brfares for both Westbury - Newbury and Frome - Newbury solves any potential problem. I have only ever had to do that once (Same day I was declined an excess actually) and was told the same as on the Newbury - Westbury train. Buy the excess before travelling. This particular case however I started my journey at Newbury when the ticket office was closed as it was at Westbury.
On a lighter note a guard joked with me that I should have used easement 700391 to buy my excess at the interchange station
There is also a trick I am led to believe that allows you to buy an Off Peak Day Return from say Bruton - Castle Cary and then get the outward only portion extended to Exeter St Davids. I'm told this trick saves alot of money and can be particularly useful for First Class tickets where an FCR/
FSR▸ doesn't exist (First Off Peak [Day] Return)