Suddenly links go to blank pages

I would try and reset your permalinks by going to Settings > Permalinks and choosing another structure and then changing them back (or check if a new permalinks structure solves the problem).

Also, check your root .htaccess file and make sure the WordPress rewrite block of code is there.

Notice that when you click on a link, it shows a https://www.paintballscene.co.uk/index.php/?page_id=27. I’ve never seen that index.php on any of my WordPress sites.

That index.php being added to the URL is the problem, see https://www.paintballscene.co.uk/?page_id=27 – that works. I have no idea where that index.php is coming from, I’ve never seen that.

The index.php was the problem and I know where it came from, though I certainly do not remember changing that parameter

Thank very much – problem solved