Had a look at the hill yesterday.

The slippage/slumping on the side slopes, above the tunnel entrance is far more obvious and well defined than I've seen it before. Didn't have a camera unfortunately. Hard to judge if it's a seasonal /vegetation thing, but a couple of things: it extends a long way up the hill, much further than the "predicted possible extent of future collapses" - which makes me think it's slippage as much as slumping. Slippage also seems to be indicated by the fact the vegetation there is very distinctly different (attributable to soil disturbance?) and, I think, there's a horizontal "break" in the surface defining the upper limit, suggesting the whole area has moved as one. All supposition, just how it looked to me.

I think the sheep are doing the hill no good at all. Maybe a seasonal thing and an illusion but there seem to be quite a network of new paths, worn down to bare soil. And the main path - a lot of it is now two very well defined distinct paths. Was that always the case?
The top of the tunnel entrance (I presume it is), that's shown here http://www.heritageaction.org/?page=heritagealerts_silburyhill_englishheritagedecision
is now a lot more visible, due to a bit of localised erosion/slippage below it, to the extent that the date 1849 is now visible (assume that's what it says, eyesight not perfect). The point being, that has changed quite a bit in a short time. Has someone kicked the loose material away, has it just eroded or have the sheep caused some of it?