"JimK" wrote in message
...

mmm I'd not thought of exactly that approach (i.e. using centre) but
was hoping to avoid digger expenses altogether plus problem of what to
with (almost) perfectly good MOT etc that will be generated - it;s a
perfect sub base for concrete after all !!

Are you sure the problem isn't a lack of compaction on the MOT?
Have you run a road roller over it?