Just to update this topic. I've now solved the issue. Having spoken to Sam at RT turns out I needed to update the firmware on the DL1. I now have a working stop / start logging button on the Dash4Pro. Basically with the updated bootloader on the Dash4Pro it needed the DL1 to be on the latest firmwar...

I've just put a new configuration on my Dash4Pro and its done a bootloader update at the same time. I've re-programmed the "set" button to become the "Start/Stop logging" function using the Dash4Pro menus. When I press the Set button, it says, "request to start logging"...