That'll be a 436, stopping off at Hartlepool, Billingham, Middlesbrough, York, Doncaster and Milton Keynes. Operated by Go North East.
All NX coaches have standard leather seats, not enormously comfy imo and took me a while to get the knack to sleeping on them. My recommendation would be to bring a scarf or cushion, get a window seat somewhere in the middle away from the door and the toilet, then shut the curtain, stuff the scarf/cushion between your shoulder and the window and try and sleep on that. You'll still wake up at every stop at least, but I've found this to be semi-effective in between.
If doing it again I'd go for Flixbus from Sunderland - only one stop before London so you'd get a much better sleep, tho coach might be a bit busier when you get on.