Great Western Coffee Shop

Coming back from Cardiff on the 18:30 Saturday 19th (advanced ticket) so seat reservation. I know it will be the old SWML so quite slow but the reservation is only to Didcot where iI should change for Taplow. Do I have to change at Didcot? Can I go to Reading where there is an off chance of getting the  previous train to Taplow (2 minutes!!). Even if I miss it, it means I would catch the same train as from Didcot plus the  chance of getting a pint in the Three Guineas.

I assume you have an Advance from Cardiff to Taplow with an itinerary that suggests a change onto a Thames Valley service at Didcot.

I don't see there being too much of an issue in staying aboard the HST as far as Reading and changing onto your unreserved local service there. Worth mentioning it to the Train Manager before Didcot just for peace of mind.

As your subsequent train is unreservable you are free to take an earlier/later service if you have to/want to. So you can make the 2 minute connection at Reading or have that pint in The Three Guineas

If you use the FGW booking engine to book your tickets, or any other 'mixing deck' based booking engine if you specify a journey avoiding one of the local stops between Didcot and Reading, such as Cholsey it will force a change at Reading. That won't work on Trainline based booking engines though.

Is there not a 'via' option on trainline based booking engines?

Booked through Chiltern as being the least worse TOC. Didn't see the seat reservtion was only to Didcot.

Yes, but trainline based booking engines take account of passing points whilst mixing deck ones don't. Specifying not via Cholsey on trainline booking engines will literally force you on a route avoiding Cholsey whereas mixing deck booking engines will just force you on a train which doesn't stop at Cholsey.