In August I booked Avanti Euston to Waverley (one way, with my return a few days later on the sleeper booked previously).
On 13th August they emailed me saying (in part)
"If your train is no longer running in the new timetable, you can use your existing ticket to travel on the Avanti West Coast service before or after your original booked train. Any seat reservations you have will no longer be valid. You can view our new timetable by clicking below.
"Alternatively, you can claim a full, fee-free refund from your point of purchase if you no longer wish to travel."
In fact my train was still scheduled to run, except for a slightly earlier departure and an extra stop at Milton Keynes, thereafter on its original schedule. What was not clear was whether in that situation my seat reservation would be valid. I decided to request a refund and re-booked on the East Coast.
Weeks went by and I began to wonder whether I should chase Avanti and whether I would be out of luck, on the basis that my original train would (probably) have run. Then at long last on 11th October they emailed me to say that they were giving me my refund, and it reached my credit card account on 13th.
Credit where it's due.