The driver of a set of motorised passenger steps, who had been tasked with servicing an inbound Boeing 757 flight on Stand 7, was stationary behind a Boeing 737, G-DOCH, that was parked on Stand 6. After the inbound aircraft arrived on-stand the driver moved the motorised steps forward and collided with the tip of the starboard horizontal stabiliser of the Boeing 737, G-DOCH. The impact, which was also felt on the flight deck, caused a member of the cabin crew of G-DOCH to fall over. Inspection of the scene by the airport management revealed that G-DOCH was parked approximately 1 metre short of the Stand stop mark which resulted in the trailing edge tips of the horizontal stabiliser protruding over the white roadway line at the rear of the Stand.