Hi, Paul, and welcome.  You'll find that the official answer from the train companies is that they're not contracted when they sell you a ticket to provide any particular means of transport, and that compensation is limited, and only kicks in when you're very seriously late. 

But I know how you feel ... travelling back from Ely to Melksham at the end of last year ... a superb run from Ely to King's Cross, a delayed service from Paddington to Chippenham Swindon, where even though the train had been announced as calling at Chippenham and Bath we were turfed out ... and a bus onward to Chippenham.  Adding insult to injury, the bus was so full that people were standing, luggage was everywhere - and we had splashed out for just about the first time and bough ourselves First Class tickets which (of course) did nothing for us on the bus!  Oh - and I really wanted to get home to Melksham - but First weren't actually running service to there at all that day.

We're very experienced in "bustitution" in these parts, and we find that when a bus replaces a train, the bus service conveys only around a fifth of the number of passengers the train would have carried.  And I think that's a pretty general thing elsewhere - there ARE exceptions such as those routes where it's well known that tickets are not checked on the buses. One particular service had a reputation of providing a great free night out.  But in general, we all find it - well - a huge waste of time.  Trowbridge to Swindon - 35 minutes by train, 95 minutes by bus.  Bus passenger's ime "worth" about 28p per minute (so says DfT) - so perhaps 16.80 compensation is due