Possible American Black Tern Eccleston Mere

There was what I thought was a juvenile Black Tern on Eccleston Mere this evening. However, having looked at the photos, they clearly show grey flanks, which is a major feature of American Black Tern, a mega rarity in the UK. The jury is still out, but there is a thread discussing it's identity on Bird forums here. Anybody like to comment here??

Also on the mere today, 9 Tufted Ducks and 2 adult Mute Swans.