Hi,

Leave Terminal 2 on the ground level( Check in) by the escalators to the Underground but exit by the revolving doors and you will be out in the pick up/drop off zone.
Turn right and you will see the Queens building ahead of you . Walk to the end of Terminal 2 ( past the place where the baggage trolleys go back into the building and turn right. At the far end thre is a green compactor in a cage. Immediately after two drinks machines you will find 2 green doors and 76 steps up to the roof, turn right on the roof and walk past various seats to the far end where you will find the viewing platform, shop, snack bar and toilets.
Opening hours 9am unitl dusk in winter (4pm) later in summer (7-8pm)

Hope this helps.