It seems to me that the National Rail Enquiries web site does clearly state the period of validity.
Right, I see what's happened here. I was on the "Your ticket" tab and looked at the "Which ticket suits you best?" section. I clicked on the link for "Anytime" and found this information under "When can I use an Anytime ticket?":
For Anytime Return tickets, the outward journey must be made within 5 days and up until 04:29 after the last day of validity; the return portion must be used within one calendar month and up until 04:29 after the last day of validity.
Which seems quite clear. I then went back to the "Which ticket suits you best?" section on the previous page and clicked on the link for "Off-Peak". On that page, under "When can I use an Off-Peak ticket?" all it said was:
The trains that you can catch and the times that you can travel with an Off-Peak ticket vary considerably depending on the journey being made, the day/date of travel and the train company that you are travelling with. Please use the Journey Planner to choose your desired time of travel and click ‘check ticket availability and prices’ to display the complete range of fares available for your journey.
Which basically seems to say: "it's complicated, just use the Journey Planner". The Journey Planner is perfectly OK if you know when you want to travel, but less so if you want to have flexibility. If it had said "or you can read the terms and conditions via the links below" that would have been helpful!
To my mind the information provided on the Anytime page is clear and helpful whereas the information provided
under the same heading on the Off-Peak page is neither!
Thanks for pointing me in the right direction.