parcel opened, goods removed and parcel resealed

I've being posting stuff through the post office for 7 years, I've never had this problem until last week. I posted a laptop down to dbn and when my client opens the package it is empty. Someone removed the contents and resealed it back up again. The waybill read as 'SPARES' just like i have always being marking all my waybills these past 7 years. How did they know that there was a laptop inside ( it weighed 2.25kg's ) and the box was wrapped in white paper printed in black and box taped properly to the brim. I get told that I have no claim because I did not insure the parcel. I've never insured anything for 7 years since using the post office which inturn use speed services, that still does not give them the right to tamper with ANY PARCEL whatsoever.