many thanks - wasn't quite what I meant though:
One of our caches on a main towpath was rendered nonviable by over enthusiastic strimming, so we has to retreat the cache down a sidepath. problem was, the sidepath was curving around coming into conflict with other caches on the main path, and, we were retreating into an SSSI area.
OK the correct answer is out in the field constantly rechecking distance to nearest cache, but it would be nice to be able to plan in advance with a map showing an exclusion zone around existing caches - and everything else!