I am grateful to +Ranbir Sahota of Vitis PR for pointing out that splitting the fare for a +Chiltern Railways journey to London at Banbury can reduce considerably the £50 off-peak return fare when returning from Marylebone during the Monday-Friday evening peak (4pm-7pm).
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.