Purchased at airport from reservations desk LHR, fare was £180~ each way to MAN and back, asked for it to be flexible. I was returning early the next day so got my inbound boarding pass at the same time.
Both boarding passes said 'Y'; no access to the north lounge at T5. When I decided to return late the same day, clerk at MAN said there is a £60 change fee. She informed me that the only flexible tickets would have J or C as the class on the boarding pass and state "Business UK" and that these also include lounge access.
Maybe it didn't say Y; maybe I had double vision at the time, maybe it was because I was already checked in, don't know, but it was a pain putting it all through expenses :P