Abandonment notice - what and how

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Hi

I am a new landlord and my tenant of 18 months absconded in the last week of July leaving the house in a mess and 3 months on his TA unpaid.
I had bought Rent Insurance from FCC Paragon who reject my claim on the grounds that they define the tenant's absence as proof of vacant possession which is at odds with how the law sees it. However the high cost of litigation makes it virtually impossible to test this in a court of law. I also have legal insurance with my landlord's house and contents cover which I am trying to invoke to pursue FCC Paragon. I keep being asked if I have issued an Abandonment notice.

What is an Abandonment notice and how do I get one?

welcome any thoughts, help and guidance

thanks and regards
ja