Thanks for the detailed reply. Indeed you are correct, simply changing the destination to one station either way as you suggest prevents off-peak return being offered as an option. That^s obviously a mistake.
Looking ahead to next time I need to make this trip - a couple of weeks away - without that off-peak ticket it^s possible to do it almost as cheaply using Advance tickets. Or if I swap the half day off work from the thursday evening to the tuesday morning - which is what I was previously doing - I can go super off-peak, or even do the advance tickets option and get the cost down to as little as ^50.50 return (off peak standard class return is ^83.50). Super off peak would also be valid* at those times, for ^66.50.
Thanks again,
Simon
* Maybe.