I suppose the replica is exposed to the air (and creatures and so on), whereas the original was preserved so long because it was away from the oxygen for thousands of years. Like a bog body. Maybe the original got buried relatively quickly, the shore can change fast in Norfolk. I don't know that it was originally built right in the way of the tide, I'd kind of assumed not? It'd be much more decayed or gone now if it'd been left there once it emerged last century, presumably.
That is to say, doesn't it all depend rather on where the circle was originally built and what happened to it, which might be rather different to the replica's situation.
is that relevant do you think