So what I did was I followed your pastebin link above, copied the code from RAW PASTE DATA, opened ADD NEW SNIPPET in code snippets, pasted the code in there and clicked save+activate.
The next/prev links still showed the next/prev dated posts even if these are not in the same category.
So then I went to the EDIT THEMES page, found the Generate Press: templates-tags.php file, located the section that corresponded to if(!function_exists(‘generate_content etc from the pastebin code, and replaced it with the pastebin code.
I felt very proud of myself figuring all this out and was sure it was going to work – but no, the links still go to the next/prev date even outside the category.
I dont mind waiting a week or so for the next update – but if there is something I have done wrong that is easy to spot and fix, please let me know!
Thanks for your patience