Re: INJECTOR warning light
I had the injector light on sporadically throughout my ownership of a GTV 2.0 TS, but the diagnostics at more than one alfa garage could find nothing wrong with it at all. In fact the engine was fantastic, a joy to drive. So I would ignore it if there's nothing actually wrong these guys are probably right that it's a loose sensor connection.
Mileage: I used to get 360 miles a tank for half motorway/half city driving, typically 30mpg which is good as the official combined consumption figure is 31mpg. Town driving only would drop it to around 330 miles per tank/27.5 mpg, so I reckon your car is spot on with the mileage. Just be happy you didn't choose a 3 litre!!