"Alex Gibson reckoned that when timber circles are a feature of a developing monument they are usually earlier , not always the case , but a useful heuristic "

Seems entirely reasonable, but timber is easier to work so not sure that it would be much of a heuristic help because it would apply to any solution?

On subject of possible solutions, today sent a package to the people who run the other monument set we discussed (at their request): Was in that country last week but didn't get the chance to go over to them. This time round I think I'll only write it up fully if they're interested (but will copy you in if/when done).

All the best G

Jon