How (and when) does Heathrow alternate arrival paths?
Heathrow has an extraordinarily elaborate runway alternation programme, designed to spread out noise as evenly between its different neighbours as possible.

Daytime runway alternation follows a daily cycle and a fortnightly cycle. Alternation starts at 6am and continues till the last aircraft departs at the end of the day.

In the morning we use one runway for take-offs and the other for landings. At 3pm – roughly halfway through Heathrow's working day – we swap over
We continue this morning and evening pattern of runway use for the rest of the week
The following week we switch completely. What we did in the evening during the previous week, we now do in the morning. And vice versa



Regarding planes over Richmond (which is situated under the approach path to runway 27L):
During the week starting 8th of April, the programme states that 27L was used for landings from 6:00 until 15:00, resulting in a lot of traffic over Richmond during the morning, but little to none in the evening.
During the week starting 15th of April, the programme states that 27L is used for landings from 15:00 until the end of the day.
During the week starting 22nd of April, they will switch over again to land on 27L from 6:00 to 15:00.
You can read much more about the programme here.