The plane pulled back at around 8:30pm, with the captain saying that everything is ready early -- and then we were just sitting on the tarmac for two hours. Literally 30 seconds after I posted this, the captain came on and said that we're clear, and we took off :-). But we definitely were sitting on the tarmac for two hours!
That said, I was surprised (pleasantly) to get an SMS and email from BA after the flight, apologising for the delay and explaining the reasons:
We wanted to take the opportunity to apologise for the delay to your flight from New York JFK to London Heathrow, we know how frustrating this must have been for you. Your flight was given delayed departure clearance from Air Traffic Control in JFK due to airport congestion, which resulted in the delay to your flight.
We don’t underestimate the disruption to your travel plans and we appreciate this isn’t what you expect when you travel with us. We want to thank you for your patience during this delay and we hope to welcome you back onboard in the near future.
I guess it was "Sheer Weight of Traffic" that caused us to be delayed :-)