Flight would have been 9W117, off 27L at 21:36 local on the day in question (30th August this year).
Curiously, although its track and altitude were displayed on Heathrow's WebTrak system following the event, all record of it now seems to have been removed, although its presence can still be inferred from the readings shown on WebTrak as it passes over the noise monitors.
Mode S/ADS-B data indicates that it was about 350' AAL as it crossed Spout Lane (Stanwell Moor).