Seabird ID help very very much needed please!

Dear All,

This series of poor video was taken recently at Porthgwarra during a gale. I cant decide if its a young gannet, a cory's shearwater, or a great shearwater?

I struggle like hell with seabird ID (and waders and warblers in fact too)!  If there are any seabird experts Id love to hear your thoughts and be ever so grateful. I suspect its a familiarity and practice thing with sea-watching.....and I have neither!