I agree however the 56 runs every 20 mins so not sure if the X36 would run, i think GNE might have the option for people for a bus to Peterlee but use the X35.I can't see either the X35 or X36 running on Sundays.
Wonder if it would be worth running an express between Peterlee and Sunderland, perhaps it could run the same route as the Arriva X21 and would take 26 minutes in each direction and would use one bus, to keep running costs down it could use an East Durham MPD.