Thanks ACX
According to Para 4, the issue for me is because the new regs now include calendar time, we cannot operate the aircraft for rent from the date 12 years after the last major overhaul, irrespective of the Cof A status, as the engine is now lifed from that date.
For CAA Private Cat with an engine within limits they can shift to 'old' Public Cat, so to speak, by letting the maintenance organisation know that the owners might want to do so.
Bit of a pigs ear, really