Differences between lesser and great black backed gulls?

Hello, I’m having difficulty distinguishing these two gulls apart. I see one of them daily now but I have absolutely no clue which one it is.

They come round singularly but sometimes pairs, have dark grey/black very long (almost lanky) wings but are absolutely massive.

This may seem a silly question but I’m not very good with seagulls, as they all look similar to me in flight.
Thanks, Ellis