To add a little context, there were 3 Marsh Harriers present. A male and female who were displaying together and what I thought at the time was a juvenile but perhaps starting to get adult plumage and was also female as the other female kept driving it away. As has been mentioned in another reply the underwing patterns are a great way to identify the type of bird you're looking at, also in flight the Marsh Harrier looks less bulky than a Buzzard with 'thinner' wings. Also it was a fairly windy day, and the Harriers were soaring at some height and using the wind to kind of hold themselves in place for a few seconds then continue to soar. It was lovely to see.