Mark Hebden - car crash.
Sid's first wife, Polly - car crash.
Greg, Helen's previous partner - suicide in the shepherd's hut.
Caroline's first husband Guy - heart attack while on the phone to Caroline.
Betty Tucker - heart attack while lying on bed, IIRC.
Did George Barford die of a heart attack? Can't remember now. I think it was sudden, anyway.
Jethro Larkin, Clarrie's dad - died after a branch fell on his head. David was cutting the branch off. Clarrie is remarkably unbitter about this.
How did Julia die? That was also a bolt from the blue, following the actress's very sudden death during a recording session.
Nelson Gabriel - unexplained death in South America.
Nigel fell off a roof - does anyone else remember this? 
Grace Archer apparently died in a stable fire - rather before my time, but I've heard it mentioned a few times.... 