He was lined up on the threshold of 27L. Two minutes departure separation, so that's why he wasn't rolling.
I was doing Arrivals on 27R at the time, and noticed something running towards the 747 from the direction of block 88 (line up from the south), my first thought was that it was a fox or other wildlife, and as I was about to nudge my colleague to warn him, the crew of BAW287 was obviously reporting what they could see to him. We actually saw him climb up the port main undercarriage leg.
The 747 shut down on the runway and got towed back.
Very bizarre.
Gonzo.