Michael Dames came up with this theory in <I>The Silbury Treasure</I> (when Mr. Cope was in short trousers, being taught by my mate's Mum in Tamworth).
It's a shame I just took the book back to the library, because there's loads of stuff about the Suil connection in it. For a long time a chap called King Suil was thought to be buried under the hill. As I recall, this idea gained favour after Charles II visited and climbed Silbury. The suggestion is that the King Suil story was essentially Royalist spin on something older.
Kammer x