I wouldn't describe my employer as a "building company", "civil engineering company" or "engineering construction company". We install temporary Christmas decoration displays including Christmas Grottos in shopping centres and other public places. Can anybody with a clear understanding of the scope of CDM 2015 give a reasoned explanation as whether or not CDM would apply to this type of work? To me, the various definitions in the regulations seem a little too vague to definitively confirm one way or the other. Any expert view would be greatly appreciated.

Hmm, according to CDM 2015 Reg 2 'Definition of construction work' and specifically (c) and (d) a temporary structure would come within the ambit of regulations - see below. Therefore your Santa's Grotto would also technically be a design and the person designing it would be a designer.

All that said, I cannot see the HSE would have any interest whatsoever in your Grotto, except of course should there be a serious incident - so nothing changes in that respect.

(c) the assembly on site of prefabricated elements to form a structure or the disassembly on site of the prefabricated elements which, immediately before such disassembly, formed a structure;

(d) the removal of a structure, or of any product or waste resulting from demolition or dismantling of a structure, or from disassembly of prefabricated elements which immediately before such disassembly formed such a structure;

You could also have a look here;
http://www.cdm4events.org.uk/
I believe that they have developed an 'app' to assist with CDM compliance.