Noise is a problem where I live too - between Brixton and Clapham North - we get Heathrow planes AND planes coming over to London City airport (which are low and loud) so a double whammy. So I have read the Heathrow website and ended up a bit of a geek on the subject!!
The planes come in over London the majority of the time (70-80% of the year) - the only time they change direction (coming in over Windsor) is when the wind direction changes to the east. Unfortunately this is often when the weather is bad so we aren't out in our gardens!
They alternate using each runways for landing - one week they use the southern runway in the afternoons, the next week its the northern runway (which affects South London a bit less). They change runways at 3pm. So one week you hear more noise in the mornings, the next week in the afternoons. There is never no noise though unless the planes are landing over Windsor instead of London - as the planes all have to join the final flightpath and they do this from all directions.
If you need more info/want to complain follow this link:
http://www.heathrow.com/noise. It's all there even graphics to watch all the planes. Happy planewatching!!