Many thanks to everyone who's replied so far.
Jim Carroll asked me where I live. I'm in Norfolk, just on the edge of Broadland. I've spoken with Peggy and with Sheila and Peter Coleman, both of whom visited Harry quite often in his last few years. However, I haven't yet made contact with Mike Yates or Bob Thomson, so I will try to do so. I've seen The Singer and the Song, but I'd be keen to know what other recordings you have, Jim.
As for the John Terry story - I've heard it but can't recall where. If Harry is John Terry's great-grandfather, it will come as a great shock to his only granddaughter, Jenny. If anyone can shed light on the story's origins I'd love to hear - I'll certainly be mentioning it in the book. Spookily enough, Norwich City play Chelsea tomorrow...