"Could anyone explain to my how some aircraft types can both be found on a permit and C of A eg PIPER L18C"
For some time in the 1990’s the CAA allowed the LAA (PFA) to put older aircraft on to a permit. It then realised it was potentially costing itself a lot of cash and decided that if an aircraft could have a c of a it had to have one and the permit was not an option. Because the aircraft, which had been accepted onto a permit, had been maintained under the LAA system it was deemed impossible to transfer them so they stayed on a permit. There is a considerable price difference between an aircraft on the LAA system and the equivalent on a C of A.
Rod1