Thanks for that information Rhuby - I hadn't thought of it in that way, but...

As a rule of thumb, you can reckon 100 years for every variety used.

This seems to assume that any one hedge was originally planted using only one variety of shrub and then patched with others. Why would that be? Given that there were, I suppose, no equivalents to nurseries from which to order a whole bunch of plants to put in, then wouldn't the farmers simply have dug up and replanted whatever came to hand to form their hedge and, later on, to patch it with?