This is quite a useful site:

http://farnborough-aviation-group.co.uk/

The best place is the Eelmore Bridge off the Fleet Road that links onto the Pavillion Road and Laffan Track. There is good parking there, and you can take photos quite easily through the fence.

The problem is that when on the predominant runway 24 you are only usually able to get photos of larger aircraft that use more of the runway length. When on the less used Runway 06 the spot is absolutely perfect.

PM me if you need more detailed directions on how to get to the Eeelmore Bridge.