If I have the train headcode, such as 1M92, and a seat reservation, how can I find exactly where the seat is, i.e. the train seating plan?
It would be more helpful to know the train service, and thus the train company and rolling stock.
For instance, 1M92 applies to three distinct train services in GB today.
One of those is the 0745 Cardiff Central to Nottingham, a CrossCountry service.
It is run with a Class 170 Turbostar.
CrossCountry publish seating plans on the website.
Thus for this particular '1M92' you'd just need to go to XC's website, open the Class 170 seating plan and match the Coach letter and seat numbers to see where the seat is.
I'm not aware of any website which will bring up seating plans automatically, even using headcodes.
Not all Train Operating Companies make seating plans publically available either.
As a rough rule - most of the long-distance operators do, but very few if any of the regional ones.