Hello Gareth, I see that tis a good idea but this is custom work 

The trick is to use orderby  parameter as rand in wp-query which fetches the portfolio items

https://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters

so for example if you are using a contain slider

you need to go to gallery-contain-page.php page template and modify 

$legrand_tax_query = array(
array(
'taxonomy' => 'portfolio_category',
'field'    => 'slug',
'terms'    => $legrand_category_filter,

orderby => 'rand'
),
);

Once again I like the idea we may include it in feature updates.

thanks

No, my mistake

'orderby' => 'rand'

forgot the quotas