The seat unit comes from Ducati Kaemna in Germany. Brilliant place with excellent gear. The trouble is that they are expensive and the current exchange rate for you guys is not good. Nonetheless the seat unit comes with everything you need - no dicking around - it fits straight on but, you lose your underseat storage. There's two versions - race & road. The road has the cutout for the rear light.
Google them and tell them Nigel Wright in the UK sent you. Alex's service is excellent.