The problem is that with such low mileage and short runs is that the DPF will not regen and clog up because everything has to be hot for it to do it.
Its a real shame that you have lost confidence and the frustation must be huge.
The MPG is not far off for short runs. Deisels are not that economical until they are properly warm, 10 miles in the summer and maybe 15 when its cold. We had a 147 JTD M-Jet that averaged 42, my run to work is 7 miles each way, but on the odd long runs 55 was easily achieveable, it didn't have a DPF, but I did have to clean the system with Wynns Gold Diesel system cleaner, which worked very well.
We changed the Alfa for a FIAT 500 Twinair, we know we'd have problems with a diesel, loads of people are. A chap at work had a Jag S Type
which he only drove to maximise economy (trying to achive the EU figures, which is impossible) and the DPF never cleaned properly and the thing choked itself, much like your Mito. I told him he's bought the wrong car, or he need to give it an Italian Tune up. In the end he changed it for a Honda Insight
I believe some are mislead by the promise of diesel economy when its not possible on short journeys, you could even say that some diesels are being miss-sold. Mind you, the TA doesn't do 60 to the gallon, we get about 40 on normal driving to work, with late 40's/early 50's on longer runs. (Which is what we expected and are happy with)
I really feel for you with the problems you've had, any modern diesel will give the same problems (The Freelander engine is currently not the most modern) and I'd advise you, if you can, to change to a petrol verson (I hope a Mito, but the choice is yours) Diesels are now very complex and petrol is making a comeback.
I realise this may not seem that helpful, and I'm not saying "I told you so", if it was me, I'd see what you can get for the Mito and get a petrol one, buy another diesel, any make, and the same problems will happen.
I hope you sort things out to get a bit of confidence back, good luck