Can confirm Costa is also 24 hour (the web is correct!).
Not too sure whether you'll want to venture over to the McDonald's at that time though!!
Better than the place getting trashed every week though!
While on the topic of the coffee bars at St Pancras, Starbucks has been closed several times in several months for various reasons. I wouldn't be surprised if it just disappears next time I'm there, leaving nothing but scorch marks on the ground it once occupied...
It was refurbed a couple of months back which is when they put the glass partitions up because it used to get freezing in there.
Lately it's been booming down there.