Thank you all for your replies and help, it is really appreciated! Thank you Girl Guide for the cremation info on Charles, I know that Crematorium very well.
I'll keep looking for some kind of record for Ethel but I suspect she was cremated. I've managed to find Audrey who died in 1990 in East Preston (just outside Worthing) and it looks like she didn't have any children so I guess the trail goes cold.
Just found it incredible that a relative who died around 15 years before I was even born ended up living less than 5 minutes from where I lived, not once but twice! Would have been nice to have had a grave to go and say hello to but such is life.