Saw a BA 757 at LHR begin a right handed turn into one of the cul-de-sacs only to be told by ground not to. (I dunno who made the mistake) Fact was, although the 757 had only just started turning a had more than enough space to make a small left back onto the the taxi way it came to a halt and was still there quite some time later. When ATC asked what the problem was they replied difficulty in restarting the right engine. Hence blockage occurred for nearly 10 minutes.