One thing I was slightly doubtful of was the Silbury I theory that the turfs were communal offerings of many a traveller and then immediately built on top off. Why would you cover a structure in turf to build on top of it straight away ? Turf isn't a great structural component unless to cap off and improve the aesthetic. I think that for no matter how short a time the turf was a closing of the initial structure. The decision to continue onwards and upwards may have been taken quite quickly thereafter. Just a thought !