From the FGW website :

09:35 Frome to Bristol Temple Meads due 10:43

This train will be started from Westbury.It will no longer call at: Frome.This is due to a member of train crew being unavailable.

Yes, thinking about it, that defeats the whole object of the service. There were others yesterday. The Pembroke Dock and Tenby to Paddington service was shown as starting at Carmarthen, so unless they bussed people to Carmarthen, passengers would have had an hour and a half wait, and a 2 hour later arrival west of Swansea, arriving London at 2231.

Can't imagine anyone inconvenienced by that will use the service again.

I was intending to use the Pembroke Dock - Paddington train - there seemed to be a minibus serving the less busy part of the line beyond Tenby - presumably supplemented by a coach or two from Tenby. I imagine it's better/cheaper for FGW to provide replacement transport than pay out the refunds for the delay caused by waiting for the next train (over 1 hour's delay, and two for London and similar flows - full refund) - plus I don't think everyone would have fitted onto the pair of 153s forming the next train! When I got home Live Departure Boards were saying the train had started at Swansea.

This seems to have happened at least once a year in recent years on this train, often on the key bank holiday weekends - not a good advert for the one-off travellers, families etc who might be using it! Is there a problem with too few drivers having route knowledge - I guess not all FGW Swansea drivers sign it, and they probably need refreshing each year, and swapping drivers at last minute could mean cancelling one of the hourly SWA-PAD trains, whcih in the big picture is probably less desirable.

True, but I assume a bus would make a significant difference to the total loss by keeping arrivals at least as far as Swansea within the hour, and Newport (along with Manchester-direction connections) if the bus made the 17xx off Carms, and halving the value of London claims...

(Edited a bit - hopefully clearer :-))