Thanks for all the responses so far.
We do use CAP437 as the basis for our helideck design, inspections, etc.
The incident we had was with a deck that is designed to be removable by crane and has removable panels as part of the helideck top surface. The panels ended up being a bit too portable, but thankfully only enough to give the crew a fright.
The SMS follow up and risk assessment calls for the frequency of similar events in the industry to be established, hence the question........