TDI 143 CVT Sudden drop in mpg

Hi, my 2009 A4 TDI 143 automatic has suffered from a sudden drop in mpg, from around 40mpg to around 25 mpg during mixed driving, which has become very noticeable over the past week or so. Currently on 80k miles and last service was on 72k 10 months ago, during which oil, fuel and air filters were changed, plus it had new engine oil and transmission fluid. Temp gauge is operating properly which I think rules out a faulty thermostat. Tyre pressure is good and I've used diesel from three different garages since the mpg dropped. I've checked brakes and none are binding. Just before the drop in mpg, I noticed that at very low speeds the revs stayed around 2k and took a few seconds to return to idle once the car stopped. This now happens intermittently. Also there 'seems' to be a bit of a whoosh or whirring sound as the revs go up from idle, not sure as I can't remember if its always done this and I'm imagining it! Also when blipping the throttle with the car in park, there's a click noise from the engine bay when the revs drop back to idle, although again, I've never listened previously so don't know if this is normal. Before I take it to a local garage, any thoughts or suggestions are most welcome. Thanks.