If the gauge is reading, it may still show the wrong temp if dirt or damage is causing resistance in the electrical cabling. In general I've found that even before my 2.6 gets to 90 degrees the heater gives me loads of warm air. The car does however take much longer to warm up in slow town driving on a winters day. I think others have hinted that the valve allowing hot engine water to the heater matrix, when you want interior heat, can fail which I agree with. The old British classic cars could just need the valve cleaning out, but the Audi may be different. As always use the screwed in bleed terminal in the steel water pipe, looking front to rear of the car, at the back right of the engine, to check there's no air lock denying water flow to the heater matrix, which I've seen happen once, because this bleed point is almost hidden.