Until such time as a "proper" name is coined, I don't see what's wrong with referring to it as Silbaby and in documents as, "A mound at the southern end of Waden Hill that has been affectionately nicknamed Silbaby because of it's remarkable similarity to Silbury Hill". For a "proper" name, what about "West Kennet Mound", or is that likely to be confused with the barrow?