"There is clearly considerable ambiguity between various bodies, and indeed individuals, ".
There is no ambiguity the correct thing is that stated by the HSE which over rides everything else. There is much ignorance in archaoelogy about this very basic simple point. The sides of an excavation must be safe. There is no set height after which shoring or other means of ensuring that collapse does occur have to be applied. It must be a major concern that this has to be discussed at all on Britarch.
Dr Peter Wardle