Simple cutting of this vid - reaching out for help

Hi,

Can someone please help me. I've been trying to simply cut this video file in iMovie for about a week and each time I fail.

I basically only want the last parts of the video, from 13:14 seconds in to the end. Every time I do this in iMovie it works fine in the library but when I export I only get the last few seconds. I've tried over and over again and it's doing my head in.

The video shows my neighbour stealing our post and I want to confront them about it.

https://www.dropbox.com/s/8r8jpwhzgo221uw/2_2014-07-01_14-00-26%20-%20Janis%20at %20letterbox.mp4?dl=0

Any assistance greatly appreciated, plus any tips on what I'm doing wrong in iMovie or is it just a corrupt file or something?

Cheers,

Stu