I arrived just as dawn broke and the rain stopped: already a local birder - Bob - was onsite and pointed out a flock of Common Scoter. He confirmed that the Lark was still present and suggested I check the far end of the Causeway - a good 800m. To keep it brief, I walked back and forth three times: two pairs of Black-necked Grebes, five Little Gulls, four Arctic Terns but no Horned Lark!
I needed to be back in Maidenhead by nine, so stopped to say goodbye to Bob: suddenly I spotted the Lark ten feet in front of us! What astonishing luck! Here are some photos with a video to follow!