<<Both aircraft were at the end of a right turn, seemed to be at very similar altitude and extremely close laterally. I've never seen any so close. Second, faster, aircraft suddenly powered up, dropped right wing and turned very sharply.>>
When I worked at Heathrow ATC I lost count of the number of phone calls I fielded from members of the public and experienced pilots saying just the same thing. I've seen it myself many, many times.
Fact is, from the ground you simply cannot tell. If Heathrow turns an inbound one off Biggin downwind right in front of an outbound 1000 ft below the phone rings quite quickly.