A month ago I had an urgent call from someone in the village to say that their House Martin nest, together with its chicks, had fallen down with the strong winds that morning. A quick inspection revealed three apparently healthy chicks, but a smashed nest. Advice is that the adults will go back to the nest if replaced in 24 hours. So, what to do? The house owner had an old plastic colander which I popped home with. I cut it in half and made an entrance slot. Elaine then tacked some hessian sacking on the inside and outside surfaces. I popped back with it, added some old nest material, screwed it to the soffit board and posted the ringed chicks back in the new nest box. This all took 35 minutes. Necessity is the mother of invention etc.
Apparently, as soon as I left the adults were back to the chicks and they fledged okay 7-10 days later. But, they're still thought to be roosting in the box and the adults are probably sitting again. You can see in the pic where the adults have added mud to the entrance hole.