Just seen the light with this one. The points to be set are all very logical in the end. (After last weeks travails I think I made hard work of this one looking for 'elements' that were not there providing me with a splendid 'doh !' moment when I realised what was staring me in the face.) A very enjoyable offering from Ploy though which didn't have me pulling my hair out for hours on end trying to solve the clues.
Many thanks to all who have tried to help me understand. I think I've got the five elements and have researched the relevant name on Google but despite (or perhaps because of) my A level Maths from long ago I can't get my brain round the necessary logic. I am always amazed at the skill of the setters and sometimes surprised by the knowledge required of solvers(as in this case).