During aerobatics, the RCP occupant (aircraft captain, non-handling) observed a screw appear from behind his ejection seat and move across the canopy as the aircraft completed the inverted portion of the manoeuvre.  He was able to secure the screw and instructed to the handling pilot to complete the manoeuvre and terminate the aerobatic sequence.  The captain briefed the handling pilot on what he had observed and directed him to initiate a visual recovery back to Linton for a normal circuit to land.  A PAN call was considered but discounted as the captain deemed that the extra priority it afforded them was not required and the situation was unlikely to deteriorate further given the nature of the incident and the planned recovery.  Of note, an inverted check had been carried out prior to the aerobatics but no loose articles were apparent.  The aircraft was returned to engineers who were debriefed on the incident and presented with the screw.