The best thing you can do is ask for an appointment with the sales manager. Take in your original order form with your spec on, ask for them to confirm the spec with the back room guys n gals (as they are the ones who inpout the order, get the comm number and track the build progress). The sales men/ladies are just a contact, they know bugger all between writing the order form and the car arriving. At least you can attempt an audit trail to establish whether the original comm number was against your order or a genuine error. You need to establish your 'new' comm number for the spec you ordered, not one similar but exact.
I know its no consoluation but a lot of August order that were week 44-48 build seem to have slipped back. Whether this is because of demand, capacity or just availability of engines or other parts no-one knows, we just like to speculate !!
I bet your order form doesnt have a delivery date on it ? A nice get out for the dealer.
Hold out for what you ordered, if theyd offered you a close spec on a short lead time when you ordered then thats one thing, but offering something at this late stage is just bad ettiquate.
If my lowly TDI order moves from BW48 now i'll be miffed, and ive suggested i'll go elsewhere, but it means buying a used/demo car or waiting it out again.