Finally

Found Jill. Hidden in a very hard to spot location in the garden. Actually not far from the front door, but so well hidden (and camouflaged) that she was invisible. Once we knew where she was, we waited to see how many eggs she was sitting on. Hunger got the better of her (it happens roughly once a day) and we found that she is sitting on 8 eggs. She has been 'missing' for a few days, so on the principle of roughly 21 days incubation which started about 4-5 days ago, we should be seeing chicks around 2nd week of July.