You're quite right that they had to show they were using the MAN slots before being able to apply for the other remedy slots (as a result, Aer Lingus (the former lessee of the slots) is facing some LHR schedule adjustments next summer).
I always thought that Cyprus Airways were leasing the original slots. This article (amongst others) suggests they are too:
Virgin Atlantic to enter short-haul market | ATWOnline
Did Aer Lingus have some others too? Just curious