I've never climbed Silbury, nor do I have a burning desire to do so. But I have climbed onto the top of The Gop in North Wales, the largest artificial mound in Wales (I think). The Gop is a huge cairn, very badly damaged by antiquarian excavations into its top. But it's still huge. I imagine some people will view climbing this as being no different to Silbury, but I mention it by way of contrast (note the fieldnotes, etc).