Good afternoon Graham and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
Just to set expectations, as you might know, the scope of our support is mostly to get our wonderful customers started on the right track to customize their site and to help them in case of issues. We unfortunately do not provide complete support for customization.
That being said, my recommendation would be to use a hook filter on the function instead of declaring the full function in your functions.php file: https://developer.wordpress.org/reference/functions/add_filter/
Also, it would be worth checking if the problem is actually still in place after the update (as we correct bugs all the time).
Let me know how that goes.
Best regards,
Geoff B.