If it is 7 am and the hotel does not allow you to check in, you still have many options to do such as Breakfast, sightseeing, spa(if opens) or just walking.
However, if it is 2am and the hotel does not allow you, what can you do?
If the walk in rate is reasonable, you might just pay for it.
But if no standard room is available or the hotel charges you much higher rate, you have to pay the high price or sleep at lobby lounge(not recommended).
Anyway, it seems to be risky if you try to check in 2am without the night reservation.
If you arrive at LHR around midnight, there are cheaper SPG hotels near airport so you could just stay there overnight, and in the morning, you can move to Sheraton Park tower.
Thanks, but i'm in a fairly complicated position in which there is in fact a way round if i got rejected (yet it's not exactly preferable given the time). However, to make is less complicated for myself and the hotel, perhaps i'll call up at 1am (when I've arrived into Gatwick), saying due to "unforseeable circumstances", I've arrived London earlier than expected and see if it's possible to check in now and then. if not, I could knock on my friend's door (obviously I'd give him a heads up now) some 1 mile down the road and get to the hotel in the morning. In the perfect world I should go pick up my parents on the very next morning at 7am in LHR (shudders), so I could also get my bum to an SPG in Heathrow (getting my way through to LHR at either 2am or 5am DOES NOT HELP the situation... but heck, i'll live)