Sorry, I don't think I phrased that very well. JC reported two diagrams formed of 2 coaches. a 150 on one and I'm guessing 158763 on the other. Could be wrong though. 'Due to a different type of train forming this service, we are unable to honor any reservations that passengers have made' is the give away when a 150 works the services. Interesting that they now seem to have a procedure RE reservations when a 150 covers. Previously I have seen them enforce reservations on a 150/1 when it covered by sticking reservation cards in the back of the seats and bending them upwards!Well 766 was on Exeter earlier so must have been a double 150 on one diagram.
It would be good if GWR ran 2 x 150 in the event of a 158 being unable to work but due to stock shortage, this very rarely happens.