Heard on the news this morning that they are migrating early - ours seem to have gone. Much to Theseus' delight, I might add, as the front doorstep has been covered in bird poo several times a day! But I have enjoyed watching them swoop around.
Found this article when I was trying to find out, and realise that we were lucky..!
www.dailymail.co.uk/news/article-2034766/Couples-shock-wake-500-house-martins-perched-home.html
People eating and drinking on the go