Ok looks like the theme needs to be deactivated then re-activated if the above is added after initial activation. All sorted.
Solved it. For reference, build the query as if your were querying multiple custom fields. This frees up the ‘meta_key’ option for use in ordering.
Example:
'post_type' => 'post',
'showposts' => -1,
'meta_query' => array(
'relation' => 'AND',
array(
'key' => 'acf_field',
'value' => 'acf_field_value',
'compare' => '='
)
),
'meta_key' => 'acf_number_field',
'orderby' => 'meta_value_num',
'order' => 'ASC'