OK that is a 3 port mid position valve , for future referance you have an open vent Y plan system, turn the heating and HW both off then move the lever on that valve across to manual and latch it then try bleeding again
I've just turned everything off, waited for the pump to turn off, I moved the MPV to manual and bled the pump, a little water tricked out. When I turned it back on its sounded great but after around 30 seconds it came back with the trapped air sound.
I'm guessing there might be off-gassing in the pipes or a more trapped air? (The boiler is set to 65 degrees due to the current temperature.) Would that temp potentially cause off gassing?