I found an account at
stonehenge-avebury.net/tourglos.html
of a Megalithic Society tour of South Gloucestershire in 2000, and although they didn’t actually visit the chambers, the way the piece is written suggests that someone had visited them in the recent past.
It says: “They are built into the steep hillside of the garden of a house in Lower Avening (at ST 879983), but visits are best left for winter. Permission to inspect is readily obtained.” [by the look of the map though, there aren’t any houses at this grid reference]
I’m not saying baza’s informant was lying. But you know how people frequently don’t know what’s on their own doorsteps. I just want to hold out some hope they’re still there somewhere.