The best news! The biggest of the three Lesser black-backed gull chicks is still alive. It must’ve fallen off the roof. I had searched the back of the house with my bins and found nothing and couldn’t hear anything, but now it’s on the roof of the lean-to attached to the back of the house. It’s moving around just fine and a parent has been down to feed it. I suspect it may have fallen into the front garden (a miracle it survived the tumble), the home owners have found it, and moved it around the back and on to the roof so it could be seen by its parents. Fingers crossed!