The best I managed in my old 05 JTD was 712 miles from a tank, but the last thirty or so were very fraught! That worked out at about 53 MPG - both on the trip computer and calculating from a brimmed tank. I've now got an 08 JTDm and the best I've managed on the motorway is around 48 MPG, but I've not had it long enough to really give it a good run. Everyday driving on swift, twisty and hilly Welsh B roads gives me between 38 and 40 MPG.
If you're only doing a couple of miles each way you need to get a bike. Or walk. Keep driving the Alfa like that and you'll knacker the engine, let alone clogging up the EGR with soot. It needs to be given a good hard drive with a properly warmed up engine from time to time, and it takes at least fifteen minutes to get the oil up to temperature.