AndreaC wrote:

Leicester Square is not bad but you do need to use the low level
corridors. These are at the rear of the train if you come from
Hounslow on the Picc Line or the front of the Northern Line trains if you
arrive at Leicester Square to make the change. There are signs half way along
the Picc and Northern platforms at Leicester Square but this is a
longer link and cuts across flows at the bottom of the main escalators for
the Piccadilly Line. Just a little local knowledge there! -

I know the interchange you mean and it is very short. Normally I would
cross to the other platform because the interchange from the front of
the Westbound Piccadilly Line platform takes you directly to the
direction of the Northern line that you wish to go.