Common Blue Butterfly , Portland Bill

Today we played hookey from the garden and had a wonderful few hours at the Bill - lots of photo opportunities but these were beautiful flirting around the birdsfoot- trefoil flowers. The two together seemed to be roosting or mating on the grass blades??
When we arrived home, however, disaster had struck - the house martins' who had built a nest balanced on our UPVC windows was in pieces on the ground - dried mud and feathers. We had watched the birds daily flying backwards and forwards feeding their babies - little faces peering out as their parents busied themselves and lots of noisy tweets- I am distraught. They are nowhere to be seen - how do the parents survive without their nest - what happens? We have ordered a manmade house Martin nest to replace it and perhaps to encourage them safely next year. Any thoughts??