I think this is a Little Egret. In mediaeval times they must have been quite common in Northern England for it was recorded that there were one thousand of them in a banquet when the Archbishop of York was enthroned; but by the 16th century they were were extinct. In recent times however they have begun to make a comeback.
Greatham Creek is tidal inlet that flows through the Teesmouth National Nature Reserve at Seal Sands. It’s well known for its seals which bask on the mudbanks at low tide. I had spent the day at the North Gare breakwater of the River Tees, looking at archaeological remains and was driving back over the Greatham Creek bridge where several people had gathered, long lenses pointing up the creek. The seals are out I thought. But no it was this egret that was attracting the attention not the seven or so seals on the opposite bank who seemed perplexed.