If u have simple heater, temperature knob didn't change coolant flow in heater at all
It only moves temperature flap in heater
If u don't like labour prices in UK
, then get some tools and change by yourself
Tools needed - 24mm wrench for steering wheel, 8mm wrench for many bolts in dashboard, 10mm wrench for heater bolts, and screwdriver
At last in original german cars
Do u want complete description, how to do that?