I had a long chat this afternoon with one of our plot-holders who helps out both with the local scouts and at his children's school.
The situation as he sees it is this:
If a group visits the site - even on a regular basis - they should/must be supervised by vetted personnel and have insurance; any contact with individuals on-site will be short-term & casual so we need not be vetted ourselves.
If a group visited on a regular basis and we helped on a regular basis - even if the registered adults were also present - we would probably need to be registered ourselves; he suspected it might boil down to frequency & regularity.
If we as individuals take a group aside, we would need to be vetted.
We occasionally have children from a local primary school to visit - something we are happy to encourage - so we need to clarify this situation. It seems that basically it hasn't changed at all for odd visits like this, when the school is "in charge" and therefore has to ensure the checks are in place.