i think i would like to go for the alfa cd player but have no idea where to get one. or do i have to get it from my friendly alfa dealler???
in a series I 156 / non-update, the dash can easily take a standard 1-DIN size (same as aftermarket) without looking out of place. i'm unsure about the UK, but some markets (countries) have used other than blaupunkt, like alpine, clarion, VDO Dayton, for example. so you don't have to restrict yourself.
in the updated dash, it is moulded for an odd-shape face, though still a 1-DIN size for the rest of the unit (casing).
photo of your dash, or confirm if it's pre-update, or updated dash?
you can try ebay or in the sale forums here at AO. will be a lot cheaper than an alfa dealer, but of course it will be secondhand, so the condition can vary.
EDIT: here's a pic of my old 156 series I dash (non-update), where i fitted an aftermarket Alpine headunit; admittedly, not well colour-matched (!) but you can see that a standard 1-DIN size fits fine; if your dash is like this, then it opens you up to the huge range of aftermarket models out there, which are going to be better value and likely better-performing compared with an original model.
the update dash is not hugely different for the headunit, but the original headunit face plate extends around the edges to meet with the lip in the dash; purely cosmetic.