If there's one thing I hate when on an InterCity train then it's the words on the station boards 'No seat reservations' or 'No seat reservations on train'. Tricky to know these days if what ever Standard Class coach you're booked into if if its an IET or HST on high-speed GWR service.GWR
In terms of how my seat reservation trains went on the 08:59 from Didcot to Liskeard and 16:31 from Liskeard to Reading, both on the 30/09/18 it was kind of like /\/\/\/.
I get out the taxi from Abingdon (due to the first bus being around 08:45) at Didcot Parkway to find the 08:59 on time from platform 3 (08:51 or 52 the "new" platform was the normal platform 1 ), formed of 9 coaches with First Class in the front 2 coaches. This meant my Coach B (seat 56) reservation was backwards - fine as I deliberately booked the same seat on the 16:31 during July as I assumed it was the same train as on the 08:59.
I get on the 08:59 to find the reservations blank - when I claimed my seat, 2 people said there's seats there. I politely said that I was autistic etc. The person and lady opposite were very nice.
In Liskeard, I had a look at Tiger and find the 16:31 is 8 coaches (therefore an HST) and not the same train as the train manager on the 08:58 said. I get on this train to find my booked seat, B56 + a mother with 2 or 3 children occupying my seat / most of these seats. I politely had to say it was my seat. She understandably wasn't happy at having to move. Both her and I had disabilities (she didn't have a crutch).
I know what you'll all think but if you were me, you wouldn't want to sit in a seat, you've not booked, on a trip of either / around 3 hours 20 minutes. Especially when you've planned a long trip for around 9 months (but 2 months for a Sunday).
If my seat was backwards, then fine but as it was forwards on the 16:31 (even though it was in reverse), it wasn't easy given how long I've planned the trip to say / think 'Don't worry I'll go and find another seat'.
Also there was no reservation ticket on my seat but was on others. I
There were a good number of positive things on both trips - my phone needed charging and thank heavens the plug socket worked where my seat was

!
The next day I complained to GWR about this regarding these problems on both trips (not in the exact words) in that basically my seat reservation not being in evidence affects my mental health! Not sure if I added the exclamation mark.
A few other things such as the incorrect information on the 08:59 between Bath Spa and Taunton I complained about.
The response time was around 2 weeks when I sent the first email and the reply to one. Given the negatives on the 08:59 and 16:31 that was worth complaining about, they sent me a gesture of goodwill as a way of making amends.
When I next go to the South West - hopefully Bodmin Parkway in May 2019 that I won't have a repeat!