this is how i bleed my M50 with no hassles

let the engine cool
open bleed plastic screw
fill radiator until bleed cries
put ignition on. You should hear some noise, like an electric motor
coolant is now gone
pour SLOWLY coolant into the radiator.
do it for like 5 minutes until bleed screw cries.
bring engine to température
you are done.
next day fill in if necessary

well I have done it only once so I'm not 100% sure my description is correct.

for coolant loss check the cabin heating radiator (PITA). Mine was leaking coolant. Easiest thing to do is to pressure test the system.