Heating Woes

[somedudebob]

Hi, Im hoping someone can assist with my odd heating issue.

i have a 2008 BKP 2.0 tdi passat that the heaters are intermittently blowing mildly warm air when set on high. This fluctuates while driving.
I have replaced the thermostat, and hosed our the heater matrix.

I also noticed on a cold night when on the motorway, the temp needle dropped from mid point, down to just above 1/4 and then slowly rose back up again, as if the car had just been started from cold, the heating was again cold at this stage

I have ordered a new heater matrix, and an auxilary coolant pump, as the water pump and belt looks to have been done recently. (Ive only owned the car 5 months and had numerous other problems already)

Ive read that when replacing the matrix that the coolant should be refilled and drained twice, Does anyone know the correct process for this? is it just a case of filling the system and draining again or should the car be run in between?

The air in the car, when set on max, varies between cold and luke warm, warming up slightly more when stationery, and getting to cold when on a motorway run. If left to idle it will get to warm, but never hot.

What is the aux water pump for?

The only error code is 00709 - Sunlight photo sensor 2 (I think its the one in the dash) so will replace this once I work out how it removes and confirm the correct part number.

Has anyone else any thoughts on why my heating isnt just hot?

Any advice gratefully recieved

Bob