Megane 1.5 DCi on 52 Plate - No heat from in car heater?

Getting no heat from the in car heater. Have checked coolant and is set to good levels, have bled the coolant screw with the engine running but no difference made. When engine starts there is a bubbling sound coming from behind the passenger side dash. Have run engine until temp gauge shows half way and tried bleeding the system with no results. Have noticed that 2 feed pipes into the cars cab, the left hand side one (from the engine) is hot whilst the return pipe (with the bleed screw on it) remains cold. Could this be an air lock problem (but should have been fixed by bleeding the system)? Or could there be an issue with the heater module in the dash board? Any ideas?