FWIW, special flight pernits can be relatively easy, or exceedingly difficult, to get. Unfortunately it seems to depend on the FSDO (Flight Standards District Office) you're dealing with.
Last time I worked on getting one to move an aircraft which had fallen out of annual inspection my local FSDO only required that an A&P (airframe and powerplant) mechanic inspected the aircraft to ensure that it was safe to fly over to the airport where the actual inspection was to be done. However, that aircraft was only 1 month expired and there were no ADs (airworthiness directives) open against the aircraft, and they made a point of saying that any flight-time limited ADs (fuel injection line security, or the Bendix fuel servo hex plug, for example) were completed *before* the permit would be issued. If that prop overaul AD is that old, the shop/mechanic (depending on whether it's a FAA certified repair station or an A&P/IA - mechanic with inspection authorization you're using) might be in for some discussions with the FSDO that covers the UK to get the permit out of them before pulling the props and sending them off.
I think that generally most part 91 aircraft run out of calendar time on overhauls (6 years on the prop, etc) before flight hours (2000), but for part 91 operations those times are advisory in nature unless mandated by AD - you should chat with your mechanic about what has to be done, as opposed to what would be a good idea to do, especially if the airframe has been sitting idle for that long a time.
I'm also pretty sure you'll find some boilerplate restrictions on the permit as well - VMC, nobody on board except required flight crew, etc etc etc).