Update to the previous message (below): the glitch with the page display (which was not a site navigiation issue as previously surmised) has been solved. We solved the issue ourselves, despite also being in touch with the Support team of the site theme publishers. Their final anaylsis is awaited, but we suspect that a recent update introduced a conflict with the way the problem pages were configured; we have found another way, which works.
We apologise for an apparent problem with the site navigation in that menu items link to a blog post instead of the indicated page.
The site theme (template) was updated today by the theme authors, which may have given rise to this effect, but at the moment this is circumstantial. We are investigating and hope to remedy the position very soon. Bear with us.