More information would be needed on altitude, type of aircraft, etc... Heathrow has been on easterlies for a few days so you may have seen Heathrow outbounds heading north. Traffic for Stansted would be quite high in that area. Leytonstone is north of London City so you probably would not see that traffic heading north or south. If it's small aircraft they could have been in transit from Biggin Hill to Elstree??