Problem with custom plugin

Hi,
I have a custom plugin on my site which sends a custom generated binary file to the end user based on how the fill in a form.
It’s all working well until after I enable W3TC. At which point something is turning the file from an 8MB download to an empty 0 byte file.

The plugin uses an endpoint

add_action('init','dd_setup_endpoint');
function dd_setup_endpoint() {
  add_rewrite_endpoint( 'demodownload', EP_ALL );
}

And I have put exclusions in everythign I can find for /demodownload/*
But I cannot figure out how to stop W3TC interferring with the file transfer.

Any suggestions would be appreciated.

D.

https://wordpress.org/plugins/w3-total-cache/