OK, all the groove talk is academic. The discs they sent me are for a BMW, but the part number (09.8961.76) doesn't exist on the Brembo database or on the "master" list that AP Brakes have, so no-one is sure what they actually are or whether they will fit. AP are sending me a returns label so I can send them back. They are stumped as to why they sent these across as replacements for the 550 brakes.
The correct part numbers for the Ferarri 550 standard drilled discs from Brembo are : 09.6802.20 for the fronts, 09.5519.30 for the rears.
So, I spoke to a few brake disc suppliers (maybe ten), and they all quoted between 3 and 7 weeks for them to get a pair in stock...then I tried amazon on a whim. They have them, apparently in stock (6 left), just type the part number into their search tool. 85 quid including vat and free delivery, so not as cheap as AP (79), but if their promise of stock is true then it's worth the extra.
Also a company called misterauto.co.uk has them for an extra tenner each. 95.54 plus 5.95 p&p