CSS menu issue

[JamesM]

Can anyone explain to me why my CSS menu works fine on certain pages but switches to a mobile version on other pages?

https://steve-morris.co.uk/

Works fine however when you visit

https://steve-morris.co.uk/properties/rent

It switches to a mobile version and the menu drops down?

 

[JamesM]

Your a star, thank you.

Something so simple!