Yes, you can always return to an in-progress check. Once you have ordered a police check, you can log out of your account and return to log in and access your check at the point at which you left it. Provided you have previously saved any entered information, no data will be lost.
To access an in-progress check, simply log in to your Certsy account, and you will be able to view your in-progress check on your main dashboard. Click on the continue button, and you will be able to continue with your check.