Spitoon
"As a controller, I would expect the pilot of any high-performance aircraft to be able to follow my instructions/requests. If the cleared level is an altitude, I would expect level reports etc to be QNH-based, if it's a FL, I would expect level to be referenced to 1013."
Spitoon, how would you manage a (fairly common) scenario where one aircraft is cleared to an altitude and the next one to a FL by approach control. Isn't the idea of the first reporting altitudes and the next FL's liable to lead to confusion regarding their actual separation?