If a design is ‘dynamic’ – that is, it does something other than its primary function (e.g. a means of adjustment, folding up, accommodating itself to change in some way), is ‘registered design’ an appropriate means of protecting it?

It can be protected by Registered Designs if what you are protecting is visible. If there is an intrinsic function that solves a problem it may be patentable but you cannot have disclosed the design before applying for a patent. If a patent is not possible, a well-crafted registered design should cover the dynamic nature of the product, e.g., through using exploded drawings. However, you should always obtain legal advice on how to register a design to maximise the protection appropriate to your work.