There is a mound near a slip road (used by taxi's waiting for customers and people arriving to pick up family and friends) at T5
, it gives a very good view of arriving aircraft landing on runway 09L, with really only one lamp post in the way.
On exiting customs you enter the arrivals area, turn right and head for an exit outside. Follow the footpath, you will pass a bicycle stand with a roof on your left (as is a Sofitel Hotel) and the apron fence on your right. Follow the footpath as best you can, in the distance is the start of the taxi rank, overhead will be the Heathrow pod shuttle service on an overhead monorail of sorts, they're small and carry a few people out to the long term car park. Pass the taxi rank and head down the footpath, the slip road will be on your left and the mound will be dead ahead. I have had absolutely no problem with the police while spotting there, you may see other spotters as well, hope the weather co-operates.