Arrivals and departures alternate on 27R and 27L when the winds are from the west, and they swap round from week to week. For example, this week, with westerly ops, 27R is for arrivals until 3pm and then swaps to departures, 27L vice versa. In the early morning both runways are used for arrivals because of the large volume of landing traffic. So this week 27L is used for arrivals after 3pm and the early morning.
When the 09s are in use, 09R will get used for departures at all times and 09L for arrivals. This isn't the case 100% of the time, with construction/emergencies/resurfacing etc they can change. For example some weeks ago there was an incident with a KLM 767 that closed 09R for a while so arrivals and departures were both on 09L.
You can find the runway alternations on Simon Nicholls' very useful page here: http://www.londonaviation.co.uk/runwayops.htm
Once you have tasted flight, you will walk the earth with your eyes turned forever skyward