Hi Malcolm these engines are so sensitive to chain stretch, I'm not sure about the max 20mm play but one of my ex lads works for the local Nissan Dealer and he reliably tells me to check the chain tensioner shaft protrusion, if it protrudes more than 10mm the chain is knackered. The tensioner is easily checked by removing the cam cover and looking down at it and measuring the protrusion. We have had similar problems on a Micra which had only done 28,000 miles which had a stretched chain but it did come up with codes for crank - cam correlation.
John Kennard