The main issue with most engines but especially diesels and at this time of year is to with the car being fully warm and at opimtum temperature whilst you are on your journey and/or when you get to your destination.
If your journey is short the engine never gets up to temperature properly and will sludge up and return poor economy. It also could be mid way through a DPF regen when you need to turn the engine off.
By the sound of things your daily commute should be long enough for the car to get up to temperature and be a decent runner. Especially as you say you do the blat down to MK once a week too.
Like many have said, Alfas tend not to be the most frugal of cars and so if economy is that much of an issue for you maybe you should look elsewhere.
Like you I am currently on the look out for a decent 159 2.4 TI in the coming months and was initially put off by the numerous threads on poor economy. However having read plenty more and done other research on the net and spoken to mechanics etc the obvious answer seems to be to get the car and very soon afterwards get it remapped, DPF removed and EGR deleted.
Everyone who has had the work done has reported vastly improved economy and better, smoother running and hasn't looked back
Initially despite being a complete petrol head I was always reluctant to modify my cars, worried that somehow I would ruin the reliability or spoil it in some way. Eventually I tried it and now I wouldn't hesitate. As long as you research whatever changes you make properly and approach the right people you will vastly improve your car and will be grinning ear to ear. In the case of the Alfa you will pay for the work you have done in terms of fuel saving fairly quickly.
The way I see it with the diesel engine, by disabling 2 of the major problem areas of the engine you are vastly reducing the chance of trouble in future. Its a complete no-brainer IMO.
As ever though the economy you get from your car can depend on so many different factors and it is hard to just say that with a 1.9 you will get this mpg or with a 2.4 you will only get that mpg etc. Having had experience of the GM 1.9 diesel before in an Astra I can honestly say that the difference in economy between the 2 engines will be alot less than you think. The bigger concern with the 2.4 would be the extra weight and the effect that would have on tyre wear.
Once again, thanks to this excellent forum I would now know to get the wheel alignment set to neutral and this would vastly improve tyre wear issues.
If it was me mate I wouldn't hesitate in buying the 2.4 at all.
Best of luck