I have so many happy memories of John's opinions and advice. A late bit of support and intervention was one of the reasons I did well in my PhD viva, and for a few years I sat on an inter-university committee with him. Several times we got into some thoroughly enjoyable technical arguments, then when we'd run out of steam, the chairman would rejoin the conversation and say mildly "err, Genghis and John, what did we just decide?". I doubt I'll ever stop missing being able to pick up the phone to him and talk over a problem.
G