Is there not an issue on the thermocouple at the top of the heater pack in the dash going wrong on these cars? I am sure that about 5 years ago my heating became less efficient. So it now is set to 24C to achieve 20C. There was a posting about pulling the top thermocouple but having to break it to get it out.. so the poster could not confirm it was the
problem but it was the solution! This thermocouple sets the room temperature and I suspect is off in my car..
I would get car up to temp (hard on a diesel in this weather!) and then feel the coolant hoses as they go through the bulkhead in the engine compartment. If they are flowing heat then it is an issue for the heater control system as it permanently flows heat through the heater matrix. So then make sure flaps moving. I did have one car where the controls became bent so that the flap did not full close off the cold air so it could not get up to the full heating output.
When doing the pollen filter put a sheet of newspaper under it as a lot of leaves/sticks/dust etc fall out onto your cleanish floor!