Know a bloke in the midlands who is FANTASTIC (I know it is a distance for you but I would urge you to contact him and see how quickly he would be able to do it - should be done within a day I reckon!). I don't know how much he would charge but I don't think he is unreasonable by any stretch of the imagination!!!
Generally pretty straight forward - did it recently on another alfa (not 156). Take out the seat and you'll probably see how the upholstery is held on. (You might also need to remove the backrest). Unclip it or whatever and you should be able to lift off the cover, replace the foam, and refit.
I guess if you've got electric seats or heated seats or side airbags then there are going to be extra complications.
Thanks for the input all. I have a GTA and it is impossible to find a drivers seat without collapsed bolsters. And it is electric backrest adjustment, and heated too with airbags in the side. I'm thinking a pro job now.