I suspect that some of the old injuries from Spa and Goodwood have really caught up with him, he broke both of his legs badly at Spa in '60 and I imagine they're quite uncomfortable now in his senior years.
Its actually a great shame that he retired from competitve racing as early as he did. He readily admits that he didn't give himself enough time to recover from his Goodwood head injuries before making that decision. He was still quite young when he called it a career, and he would have been something to see in a Ford GT-40, Lotus 49 (in the blue livery of Rob Walker of course).
Fax