Great Western Coffee Shop

Classic example to use for illustrative purposes.

2nd June, Melksham to Pewsey for a day's seminar (proposed)

Out on the 08:52, change at Westbury, arrive 10:18
Return on the 18:02, change Westbury, back into Melksham at 18:47

Return fare (not looking at railcards, set - £32.60)
or
Return fare Melksham to Westbury - £3.90;  Westbury to Pewsey - £8.50 - total £12.40
so
Saving £20.20

Is it allowed?   

YES if the train you use calls at the station where you switch from one ticket to the other, AND each individual section ticket is valid for that section on that train.

Additionally, you can join up between a season ticket or rover ticket, and another ticket which is not a season ticket or a rover, at a station that the train passes through but does not call at.

Almost every manned ticket office (*) of a franchised operator is required to sell you all the tickets in a combination on request - even for services not starting at that station, and writing out a list of what you need to had the ticket office / work through with them is a good idea.  If you are starting from a station where there are no ticket selling staff on duty, get the first 'leg' from the ticket machine if you can and buy the rest from the train manager.

(*) - Exceptions are around such as Paddington after 9 p.m. where the ticket office is not obliged to sell you tickets for the following day - and indeed have refused me looking to buy ahead because I knew I would be in a rush

Another as you say classic example of our crazy fares system.

Interesting that there are no Day Return fares for this journey, either Standard or Off Peak.

Pewsey has for many years been a "London facing" station. 50 years ago there were but 2 departures each way per day - at 07:56 and 09:00 to London (arriving 09:47 with a change at Reading, and 10:25), with return trains from London at 17:03 and 18:30 - both through trains, arriving Pewsey at 19:04 and 19:54.   

Even today, you'll find that if you want to travel west before midday, you have a choice of the 08:09 and ... err ... no choice, whereas for London you have trains at 06:18, 06:33, 07:20, 08:10 and 10:19, with a "chicken and egg" suggestion that they're that poor because not many people use them where (talking to people) suggests significant interest in living in Pewsey Vale and commenting to Bath or Bristol. If only the one morning journey weren't nearly half an hour longer than it needs to be - longish wait for a connection at Westbury onto a s-l-o-w onward service arriving well after 9 a.m. and would only work for flexible employment, and rubbed in for people travelling home late by a return  service that's 24 minutes faster.

Interestingly, there are day return fares from Pewsey to Bristol Temple Meads - £18.10 which looks like a snip against the £32.60 from Melksham.  An off peak return from Pewsey to Salisbury is £19.50 - at peak times it's £69.20 which you would be asked to pay if you were commuting from Salisbury into Pewsey, but not if you were commuting Pewsey to Salisbury.

The Pewsey [Vale] Rail Users Group that we're looking to help seed may well wish to take up some of these fare issues with GWR and we can make introductions, and indeed the 4 hour gap in the morning is already on the agenda with movements being made towards narrowing it.   And I can see other commuter flows growing as services are updated with new trains and different running times.

It needs to be remembered however you have to ask for the combination of tickets - the ticket office is under no duty to look for you. There are in some cases multiple splitting points so it could take hours which would annoy those travelling on the day. If you want to know the best split ticket options you'll have to do the research yourself.