It's no great secret! We arrived in the Ockham hold yesterday afternoon having flown from the western USA. Winds and levels had not been friendly, and we'd pretty much used our contingency fuel by the time we were approaching the hold, with an unknown number of aircraft ahead of us. We were aware of the single runway operation and, like most, I suspect, were keeping a careful eye on Gatwick 'just in case'. We'd been given an EAT that we were happy with, although it left us with little fat.
The loss of the second runway naturally changed the picture somewhat, and meant that our EAT was likely to slip into unacceptable territory. We thus set in motion, through Company, a diversion to LGW and informed LHR ATC that we would be diverting in a very few minutes if there was no change in the situation. Approach (bless their cotton socks!) told us that 9R was just about to become available if we were prepared to go for it, and we accepted. We landed still with enough fuel to divert, though it wouldn't have been a popular course of action!
The point is that you don't get yourself into a hole you can't dig yourself out of, and you set things in motion before they become imperative so that any last-minute snags can be ironed out without it becoming a panic.