Oh, having had only two real word flights due to weather and some bug I caught I had not realised the app does not allow me to delete the videos.
In that case I will connect to iTunes
See if that App lists its files when I click on its icon. Some do
If it does I will select the files and delete them within iTunes.
If the above fails I would uninstall the app and agree to delete all its documents as well. Then re-install it and start afresh. This would be a pain in the arse though so would only do it when space was beginning to run low on my iPhone.