Really? Should someone be forced to move when they've taken up a seat that is empty on departure from the station from which the seat was booked?
If I got on the 158 at Westbury and saw a 'Westbury to xxx' seat reservation that no one had taken up I'd assume they were a no-show and the seat is therefore available. If the service is busy I wouldn't be giving up the seat if that person turned up later in the journey. Unless the reservation slip/display had the name of the passenger on it. Then I'd have to move else I'd be in breach of Byelaw 19.
One of the most annoying things I see is the amount of reserved seats that don't get taken up. Why should I be inconvenienced if someone else has changed their travel plans.
If your plans change and you are travelling with a flexible ticket then you should try to amend your seat reservation. If that can't be done or you're not happy to pay any amendment fee that may be due, then sadly, I'd say, tough.
There is no requirement to take up a reserved seat at the first station from which that seat is reserved. The seat has been reserved for the whole journey as listed on the reservation label or display, and a passenger can choose to take up their reservation at any point in that journey.
Of course, in this example, you would be quite entitled to sit ina seat that has no one sitting in it, up until the point that the seat is 'claimed'. If you are travelling to Salisbury then you will vacate the seat before the passenger sist in it in any case.
Sitting in a seat that is clearly marked as reserved always has an element of risk that the rightful occupant will turn up at some stage of the journey. They may be delayed getting through the train with luggage, they may go to the toilet, or on long distance services they may be at the buffet. They may even have simply decided to join the train later, which is perfectly acceptable on most tickets.
The former has happened to me, and it took 3 or 4 stations for the crowds on the train to thin out enough for me to store my bags and get to my seat. I was happy enough for someone else to use it while I couldn't, but as soon as it became accessible I expected them to move. Coincidentally, this was on the same line as the OP's issue.