Search entries by location or name
I managed to solve it finally, by starting from what was suggested in the comments. Basically to get the filter to work in Craft I've added action="{{ url('developments') }}" to the form class, which keeps the url on the same page, and then on the  tag I added a name="county1" and some javascript to get it to reload when it's selected using onchange="this.form.submit()"

Then in the results below I set a query variable using {% set query = craft.request.getParam('county1') %} and then created a second variable called queryFilter to search the correct category, based on the selected field {% set queryFilter = craft.categories.group('developments').search(query).order('title asc') %} which to print out the results is then referenced via a for loop {% for category in queryFilter %}

So there's quite a few steps to get it to work, so it's not something that's obvious from the Craft documentation, which doesn't have hardly anything on this subject that I've found.

Here's the code as whole so that it help makes sense to someone trying to do the same type of thing:

&lt;form class="form-inline" action="{{ url('developments') }}" method="get"&gt;
  &lt;div class="form-group"&gt;
    &lt;label for="county1"&gt;Location&lt;/label&gt;
    &lt;div class="selectwrap"&gt;
      &lt;select id="county1" class="form-control" name="county1" onchange="this.form.submit()"&gt;
        &lt;option value="all"&gt;All&lt;/option&gt;
        {% for category in craft.categories.group('counties').order('title asc') %}
        &lt;option value="{{ category.slug }}"&gt;{{ category.title }}&lt;/option&gt;
        {% endfor %}
        &lt;/select&gt;
      &lt;/div&gt;
   &lt;/div&gt;
&lt;/form&gt;

&lt;section class="developments"&gt;
    &lt;div class="container"&gt;

        {% set query = craft.request.getParam('county1') %}
        {% set queryFilter = craft.categories.group('developments').search(query).order('title asc') %}

        {% if queryFilter %}
        {% for category in queryFilter %}
        &lt;div class="development-block"&gt;

        ....markup...

        &lt;/div&gt;
        {% endfor %}
        {% endif %}
    &lt;/div&gt;
&lt;/section&gt;