Passing URL arg with permalinks

I have a web site <http://www.wessexmc.org.uk/&gt; which uses quite a bit of custom code, implemented as a plugin. These use parameters passed in the URL. e.g. <http://www.wessexmc.org.uk/?page_id=234&year=2015&month=2&gt;

This all works fine, but I would like to change to name based permalinks to make the URLs more readable.

I am trying this on a test version of the site. e.g <http://test.wessexmc.org.uk/?page_id=234&year=2015&month=2&gt;

When I change to named permalinks the base page is <http://test.wessexmc.org.uk/hut/hut-calenda-and-booking/&gt; which is fine but <http://test.wessexmc.org.uk/hut/hut-calenda-and-booking/?year=2015&month=2&gt; gives a 404 page not found error.

I read I needed to tell wordpress about the parameters (although that was not necessary before). So I tried code like this:
add_filter(‘query_vars’, ‘add_my_var’);

function add_my_var($public_query_vars) {
$public_query_vars[] = ‘month’;
$public_query_vars[] = ‘year’;
return $public_query_vars;
}

But that made no difference.

Anybody help with what I need to do to make this work?
Thanks