This may seem like a daft question but when you split AP tickets on HT and EC what happens to the reservation at the split point. In other words would you be safe to carry on sitting in the same seat in the knowledge that nobody else would have reserved it or is there the possibility that a passenger boarding at the split point could have a new reservation for the same seat? Are reservations on HT and EC for the whole journey even if the person is only travelling part way?
A booking office can try to ensure you get the same seat throughout.
Trainsplit.com will automatically try to do this, where it's possible (It's not always) - though I am unsure exactly where it's at at the moment, but if it doesn't give you the same seat now, it will be more likely to do so in future, and in the long term you may be able to choose your seat.
If you book individually on a website like Hull Trains' own site, you may end up playing musical chairs throughout the journey.
With East Coast's own site, if you are travelling on their trains, you can select your seat, so you could pick the same one each time, but this does not work if the ticket is the last one in the quota.