WordPress - redirect after login from event registration or contribution page

Overview

If you create an event or contribution page using a profile that allows or requires CMS user account creation then the event registration / contribution page adds the message 'If you already have an account please login before completing this form.' with 'please login' linked to the login page.

Current behaviour

On Drupal, the user should be(*) redirected back to the event registration / contribution page after logging in. On WordPress, the user is left on the WP profile page after logging in.

Proposed behaviour

On WP, redirect back to the event registration / contribution page. This improves the user experience when registering for an event or contributing by taking them back to the page that prompted them to log in.

(*)Drupal7 behaviour is broken but that's a different issue...