Are you able to name the aircraft type/engine model on here or is that giving the game away too much ! Regardless , what does the approved maintenance schedule for this aircraft say about engine TBO or O/H periods in general ? What is the approved maintenance schedule ??
I have no idea what NATS or their AIS have to do with Lycoming TBO's or even if Aeronautical Information Circulars are even legally binding or are approved data....get the said engineer to produce this type specific AIC and go from there !
VK