I have a 2000 D 2 TD5. I have noticed that over the past 2 months the heater has been less effective. I have drained cooling system, put in new coolant and bled the system via the bleed screw in the top hose.

Still I can hear bubbling thru the heater and the heater is not very warm at all.

Could it be that the water pump is now not pumping enough and not noticeable due to the cold weather. Temp gauge still reads around 1/2.

Any thoughts please.

Did you fille the system the right way?: tank raised above the bleed hole?, if not remove the bleed screw, unclip the tank, lift it as much as you can fill untill clear flow comes out of the bleed hole, refit partially the bleed screw, put the tank back, start the engine and keep revs above 1500rpm untill no bubbles are coming out, tighten the bleed screw top up the tank and tighten the cap well... that's the good procedure otherwise there might be an air lock in the system...the water pump when fails it's usually leaking but if it doesnt work well the coolant will overheat as well

Yep I did that procedure for filling as you have stated.

isnt the top hose solid after a longer run?