Once arriving at Paddington the train only allows you to unload passengers if you open the doors opposite to the side the platform's on.
LOL, ignore my first reply - I realised afterwards that you had put the exact service number (1P51) in the thread title. So, I had a go at this service. Pulls into Platform 3, and yep, you have to unlock doors on the left and ditch the passengers into the six foot. At least there is no third rail there.