Under COP9 Tax Investigation guidelines, you must send CDF back either accepting the offer or denying the offer. If you do not send the form back, HMRC will take it as a denial from you. Fraud investigation service can only close the investigation if they accept your explanation along with the CDF form. Not submitting CDF is not an option. Accepting tax evasion and submitting the CDF offer form will give you immunity from prosecution. If you do not send the CDF form, HMRC will start investigating further and may commence a criminal investigation with a view to prosecution.