Weird Heater Problem - Could You Help Me Diagnose?

Afternoon All,

I have a weird heater issue in 63 reg 1.6 tdi sport with climate control. The dealer wants £100 just to get the laptop out and is trying to suggest its a software update (not convinced), so here's hoping you can help me?

* when cold on start up, all 4 vents work, blows cold, but at about 70% air intensity (when the fan is turned to max), so its not blowing as it should. Window demister also blows fairly strong (albeit cold air).

* start to drive, engine gets up to temp from cold as usual and stays at normal level - as shown on dash

* the cabin is comfortable enough set at 22 degrees on the climate control on the dash with the blower set at about 25% blow speed. You can drive all day and the cabin is plenty warm enough in the cold weather. 

* However, turn the blower up and centre vents die off pretty much completely, you can hear that the fan motor making a faint whirring noise but its still working and blowing, but faintly now through end vents and centre vents almost nothing. I have tried adjusting vents etc, and nothing. Demister hardly blowing.

So I have heat, but things take a turn for the worse when you try and increase the blower. 

I have read up a bit on here and know that this seems to be a problem. There is the thermostat, heater matrix, filters getting clogged, broken blower motor, and blower motor resistor. I am not an expert but I would have thought the thermostat is ok as the engine gets up to temp. Could it be the resistor? Its weird that the centre vents pack up though when the blower intensity is turned up (after it gets warm) yet the side vents do still work a bit. The centre vents dont struggle as much when cold though.

Could anybody help me understand what the problem is please?

Huge thanks

Julian