The page does contain a header element…
…well sort of. The opening tag is broken:-
<header id="masthead" class="site-header" role="banner" style="background-image: none; <div class="col-full">
It does not have a > on the end.
May I introduce a very useful tool, the html validator:-
https://validator.w3.org/nu/?doc=http%3A%2F%2Fwww.brewerybay.co.uk%2F