Hi, first, thank you for taking the time to reply and being so helpful including the link and photos. I know absolutely nothing at all about car maintenance, and wouldn't know how to use a multimeter to check the power. Although I have used them in other situations I wouldn't know where to start or what I am looking for in this instance.
I have seen on the "new" radio there is indeed a fuse at the back, and see it in your pic. The radio I have in, was a temporary replacement for the original "Nissan" Tape player, it was bought online at about £12 new, I was always planning on replacing it. This has just pushed me to do it now, rather than later.
I had this radio fitted at a local fitting place, so I know the wiring was also updated to "ISO" so hoping that with the use of a YouTube video, it will now be a simple swap.
I will update this afternoon when I get chance to attempt the swap. If the "new" radio is also dead, then I think it will have to go to an expert.
Thanks again.