Hello, Well there are 2 official viewing areas. The first is on top of the Multi-Storey car park attached to terminal 1 (Level 13 to be precise) there is a small shop full of aviation magazines etc and a small snack bar. It gives a good overview of the main T1 and T2
area but you cannot see T3
nor actual touchdowns on 24R/take-offs on 06L as they are obscured.
The second area is the official aviation viewing park which offers unrestricted views of both runways. You can also see part of T1 and T3
but not too much. There are a couple of static displays including Concorde, a bigger shop selling magazines/models/timetables and radios etc and also a bigger snack bar.
I do like the main aviation viewing park as I prefer to see take-offs and landings as opposed to taxiing around a terminal. Also for me the car parking is much cheaper than at the actual terminal. However I expect you'll need public transport!
This link gives details of the bus link from the airport Station to the viewing park.
Enjoy your day