Where can I find a list of these easements please? I feel that this would be very helpful in answering the question.
For tickets routed "Any Permitted", this is documented in the
Routeing Guide instructions on page 3:
An "any permitted" ticket cannot be used for travel on a route not listed in the Routeing Guide for which a higher priced route specific fare exists. It can be used on any route not listed in the Routeing Guide for which a lower priced route specific fare exists
I am struggling to find if it is explicitly stated for other routed fares, but NRCoT page 12 states:
If you make a journey by a route that is not valid you will be liable to pay an excess fare. The price for this will be the difference between the amount paid for the ticket you hold and the lowest price ticket available for immediate travel that would have entitled you to travel by that route
Arguably if a lower priced fare exists, the more expensive fare should be deemed valid. But if it is not, the worst that can happen is the customer is issued a zero fare change of route excess.
You are quite correct, that is why it is a good idea to get an itinerary at the point of purchase.
Obtaining a through itinerary for the journey being made can be useful for all sorts of reasons, including demonstrating that the trains did exist at the advertised times when the contract was made, and that the ticket(s) were valid at the time the contract was made, even if changes occur between the date of purchase and the date of travel.
As I said elsewhere:
...I have been made aware of an increasing number of instances where a passengers intended itinerary is no longer possible on the day, due to late notice changes to the timetable, and this has caused all sorts of hassle for customers, so my advice to anyone who wants peace of mind is to book their intended journey in one go if possible. This means the customer can easily produce evidence of a contract for a through journey.
If it's not possible, or the person making the booking and journey is confident in being able to resolve issues, then by all means book each leg separately.
While showing such an itinerary does not guarantee a hassle-free journey, and does not guarantee that the relevant train company won't continue to argue in the event of any dispute, it's certainly good evidence to have.
A good retailer will back a customer up, in the event that a train company claims the ticket was not valid for the journey for which it was sold.
The sort of disputes that can occur can be quite unbe