I note Liz that you have put up the Jacobi Jayne nestboxes for the house martins. I put those up initially but didn't get anything in them but then I purchased two 'conjoined' woodcrete boxes from C.J.Wildbird Food and they have proved more successful. I now have a pair of house sparrows nesting in each half! Anyway it might encourage the house martins to nest. I think their numbers are really down in my area this year. A few years ago I had 21 natural nests under the eaves of my house now I haven't even got a prospecting bird. Its awfully sad. By the way the woodcrete type box which you and I have put up will be used by roosting birds in the winter such as wrens and of course they are easy to take down to clean at the end of the summer. I give mine a good 'gentle' wash with dettol and water.
If you are interested in purchasing the specialist nest-boxes described then visit the following websites:-
http://www.jacobijayne.com/
http://www.birdfood.co.uk