Bulldog IAS
I think Mandator has hit the nail on the head.
Original RAF Bulldog engine pool must be getting awfully long-in-the-tooth. ISTR the engine in XX553 has more cumulative hours since factory new than the airframe itself with multiple overhauls during its 40+ year life. So, despite regular service, the engines must be getting a little "wheezy" by now. On the other hand, props should be fine as they were all replaced following a Hartzell AD in the late 90s.
I know a couple of Bulldogs have had more recent vintage engines fitted - Bulldog XX554 and one of the Ultimate Highs. Perhaps Cim Jartner and Greeners, respectively, could provide a before-and-after comparison before we all rush down to the Lycoming factory sale in pursuit of an extra 10kts.