I spent ten hrs yesterday listening to a marsh wrb at Sammy's Point, Easington. I have singing reed wrbs on my local patch that I hear on a daily basis. That video is a reed wrb singing. Can discuss the appearance till the cows come home but doesn't alter the song
I'm hoping you don't think that I believe the Gosport bird is a Marsh, which it clearly isn't. I was just comparing the obvious differences from the Otmoor Marsh.
For the record, it's a Reed Warbler.
All the best