Mobile menu toggle

Hi
I think I must be missing something simple. I just can not get the mobile menu to open up the main menu.
Created a set of menu links on main menu
Put the main menu block in the Menu region and the mobile-menu-toggle block in both the menu and brading regions (not at the same time).
I have added CSS into the subtheme so that the Main menu appears on the desktop and the mobile-menu toggle on the small screens.
BUT when I click on the Menu on the small screen nothing happens - the link shown is Javascript:;
I am I being particularly dense - what must I do to get it to work?
Thanks for taking the time to read this