Thanks Bigyin for the link, it has some great advise there,I need it ! I only found out I had olive trees last week  ::)
As for concrete mix ,your right it would work out more expensive but, can fill a hire car up with bags,I cannot really have chippings/dust/sand delivered loose as it is a shared track "driveway" and doubt very much if they would get a truck up to the cassita, and I only intend to use for footings  on a small-ish almacen so would not be ridiculously expensive .
Thanks again for your help
Phil