how can I fix poor fuel consumption?

I have a soon to be 3yr old skoda superb elegance 2l 170bhp automatic diesel car. I recently had a major fault due to a leaking oil seal into my turbo, which led to the car going into limp home mode. I have had the turbo and diesel fuel particulate filter replaced, however, I am still experiencing issues. If I am lucky I get 41mpg consumption, but normally I am around 25-31mpg consumption, and this does not change even if I am doing a steady 20-30 miles an hour. The poor consumption is normally followed by the onset of my fan continuing to run on after I have got out of the car, sometimes coupled with a rather burning smell which the garage have said is the car purging itself..but to do this every few days is surely wrong! It went back into the Skoda dealership, however, despite reporting this they cannot find anything wrong with it...most likely because the issue is not big enough to register on the on board computer, unlike when my turbo went! The car has had a good blowout run and even tried taking it off automatic and doing it manually to see if this would help. I have also noticed that at 40mph I am sometimes in 6th gear..so wondering if it is an issue with my dsg gear box. it is going back into the dealership for a window switch replacement on the 22nd feb, but I need to sort out the consumption issue. Any ideas would be welcome so I can get get them to check it out as I can't keep driving with such poor consumption.