It took a long time for the ISP (Gigaclear in the UK) to acknowledge what I was telling them and get an explanation of what was going on. It seems they (along with other isps) took a decision to implement CGNAT. {Carrier Grade NAT: https://en.wikipedia.org/wiki/Carrier-grade_NAT } This allows them to allocate (putting it simply) a single public IP address to multiple customers. The ISP did not consider it necessary to advise customers, since
I guess that puts me in the 5% bracket!95% of customers won't notice
One minor effect is that customers can NO LONGER USE PORT FORWARDING on their router {effectively my router would be behind a 'double nat' and all that encompasses, especially as it is ISP controlled.
So, if you want @TMSupport to look at your NAS, you are out of luck.
There are ways to overcome this but, for now, I have chosen the cheapest, which is to ask to go back on a dynamic IP address direct to my router. I could get them to put me on a static IP, but I have no guarantee that they won't try it on again. No doubt the intention is to put all their customers on CGNAT. One possibility is to use a VPN to bypass CGNAT and use the VPN address to access my site, but that obviously will incur expense in one way or another. Cannot readily change supplier ISP either as Gigaclear is the only Fibre supplier in the area. (Not even FTTC - still in 20th Century here, or maybe 19th)
So, can anyone suggest a reasonable way to implement a 'proper' VPN for my internal network to the outside world? (and back again)