The trick is to buy a Network Railcard (£28 per annum from Banbury and stations south thereof) and use the trains south that call at Banbury post 10am (ie the 0912 Snow Hill departure onwards).
The Birmingham to Banbury Chiltern Only Anytime Day return is £13, and the Network Railcard Off-Peak Return from Banbury to Marylebone is £25.50 making a grand total of £38.50, a saving of £11.50. NOTE that you cannot use the 1715 Silver Set back to Birmingham as this does not stop at Banbury.
So, a minimum of 3 journeys a year will save you money and give a better journey pattern and, of course, you get the additional benefits in the London & S-E area that the Railcard brings.
+Ranbir Sahota of Vitis PR emailed me in August (apologies for the delay!) to add:
I have now used this journey combo a number of times now and would suggest that travellers carry a set of used tickets with them in their rail card wallet, as sometimes the ticket office struggles on finding the fare!