Just a quick update and to put anyone's mind at rest. Had the wheels off today and there are huge recesses in the back of the wheel where the disc retaining bolt head can easily sit. Have to say though, the Alfa original disc retaining bolts only sat proud of the Pagid discs by a mm or 2 so no big deal anyway. The front pads (again Pagid) also definitely only have one wear sensor as per the originals.
Only problem I had (actually that's a lie, it was a comedy of errors but that's for another thread!) was I couldn't get the pad retaining pins out. Had a bit of a struggle on my 5 year old Touareg but this 2 1/2 year old 159 is much worse. I've sprayed some GT85 penetrating oil on the pins to see if that helps and I'll try again another time.
Anyone have any other ideas?