My 2000 TD5 has developed a issues with the heater in that I can hear water bubble noises on occasions and the heater is not all that hot even when dialling to Hi.

Is this a sign that the water pump is becoming inefficient and not pumping enough water around?

I changed coolant and bled the system by holding the overflow tank higher when filling and also undoing the bleed screw on the top hose thinking that there was an airlock. Perhaps there still is !.

Temp gauge is normal ( just over 1/2 way) as it has for 9 years.

Any one can help out there.

There's air trapped in the heater matrix, how did the air get there as the as u have a closed circuit system, then I see you have changed the coolant.
So if u didn't have this issue before u changed the coolant then the system requires draining, refilling slowly and bleeding again following the approved method for the Td5 to remove the air in the matrix.
But if the air was there before the coolant change u may have an iffy head gasket.