Put money on it being the AQS sensor in the scuttle air intake. Exactly the symptoms I had, about £95 from Volvo.
PIA to get to it as the wiper arms and the scuttle needs to be removed, then a bit of contortion where you really cant see what you are doing to twist to remove the sensor.
Once replaced heating was back to normal.
I would have thought the AQS would have thrown a fault code but may be wrong.
Hope it helps
