<<some of them will be coming off the Lambourne stack on their way to the city and the turn for Heathrow. >>
On the day in question Heathrow was on easterlies so the LAM inbounds would be well north of Leytonstone at stack level.
Nicholas. Speeds you are seeing on that web site are probably ground speeds - see my earlier posting. Airspeed on final approach will be close to 160 kts from about 10 miles to about 4, then it rapidly decreases. However, if there is a 20 kt wind blowing, the 160kts airspeed becomes 140kts groundspeed. If the wind is coming from a direction other than straight down the runway the ground speed will be faster, but still less than the airspeed.