Child page shows in the main menu

Hi, I have 9 pages that appear in the top menu and just one additional page set up as a child to one of the main pages. Worked fine until today as it has decided to sit on the main menu too. It is set up correctly, in the edit page it has the correct parent. The permalink shows it to be a child of the parent. In the pages page it shows underneath the parent page with the 2 dashes in front of it indicating it’s a child of the page above.

I have added a new test page and set that up as a child to a different parent and the same happens to that – it shows up on the main menu, not as a child

I’ve viewed the site with Chrome, Firefox and Safari but they all show the child in the main menu!

Any ideas?