I have found sending a complaint in writing by postal method gets the best response from most TOCs
Ironically, it takes about the same time as email, by the time you take into account their initial response and having to email back that they may have misread and misunderstood the points raised
So far the best response by email is from Arriva Trains Wales
They went out of their way and even met me at Cardiff Central station whilst on my ALR to discuss the content of my email
First TransPennine Express would probably be next
Their first email was a little vague, but they responded to my second with the exact information I required, including an attached copy of the replacement seat reservation
Virgin Trains did respond by email, but advised me to contact someone
Sadly that email didn't include that persons email address, it assumed I knew what it would be!