Use personal service option. In a previous role I had similar issues where debtors would not accept legal letter.Two tips:
Engage a solicitor.
Arrange an alternative means of certified delivery.
Love that suggestion.Send it in a nicely packed small parcel with DPD.
Ideally recycle a small box from any leading online retail store. Valentines day might be an opportunity.
Dpd will show delivery and a gps mark of the location of the delivery.