Ahhh, got it sorted.
The theme I’m creating was based on underscores theme, its css included the following:
@media screen and (max-width: 600px) {
.menu-toggle,
.main-navigation.toggled .nav-menu {
display: block;
}
.main-navigation ul {
display: none;
}
}
I’m guessing you use the same class names?
Anyway I’ve deleted the above and it all works fine now, sorry to waste your time.