I suspect those in the know can't give you chapter and verse here due to worries about Big Brother. The weather in London was OK and there's nothing odd going on with ATC, so you'll probably find this is down to something like the original aircraft being found to be unservicable not long before departure, resulting in a scramble to find a spare aircraft and perform a late aircraft change.......I doubt the two delays (yesterday's and today's are connected).
