Thank you for your replies.
My router does NAT and connected to the router is my Linux box Firewall, to which I connect, so NAT on that as well. Confess the firewall was set up by my nephew and is beyond me, https://www.grc.com
says I am in true stealth mode, but If Skpe is advertising itself from within my system I appreciate that will make no difference.
I did read lots of things about NAT stops you becoming a supernode, but they were all quite old messages (2005-2009) so wondered if Skype had modified their software.