Hi Peter,
All fields in all meta boxes are merged and displayed in REST API responses. I see the link https://www.pdpa.co.uk/wp-json/wp/v2/events shows some fields. Can you check that fields come from all meta boxes?
In case there are some fields are missing, please check your code that registers meta boxes. Make sure there's no conditions like:
if ( is_admin() ) { // Or any similar condition
add_filter( 'rwmb_meta_boxes', 'your_meta_boxes' );
}