Gnad - the Seasprite as I understand it was orginally selected because it had a footprint small enough to operate from the proposed OPC vessel (later cancelled). Effectively they were trying to shoehorn Sea Hawk type capabillity into the smaller airframe - hence the requirement for the systems to be designed from scratch. Defence have never provided an adequate answer as to why the Seasprite wasn't cancelled when the OPC was - probably mostly bureaucratic inertia.
