easy to change but as said, pain to bleed. pretty sure its the same as all 6 pot bmw. Thermostat is located front of the motor near top, it'll be black with 2 plastic clips on hoses

certainly sounds like thermostat but could try bleeding it again, The stats aint cheap. If not buying from main agent get a Behr stat, they are OE suppliers, think around £45 from GSF