In reply to Roger’s enquiry a very useful site to visit is www.swift-conservation.org this is the new website for ‘London Swifts’ an excellent conservation group and this site is packed with useful information and also has links to other sites. From my own limited experience I would suggest the ‘built in’ option is far superior to external boxes. One of the biggest problems is attracting the bird’s initially; white plastic facia boards / eaves are great to maintain but the longevity of the material gives little opportunity for access to the wallboard where Swifts like to nest. Upon close observation of the birds you will note they are attracted to dark cracks and holes particularly in stone / brick or roofing tiles; the built in boxes emulate this. A little tip is to keep all access closed until at least May (sorry Sparrow fans I’ve nothing against the birds honestly!) Hs Martins are different story the external boxes are great and I think superior as you can place them where you want them I make my own from a quarter of a 8 inch ball from a ball-cock. The home made boxes stand off the wall a little more than a naturally bird built nest and therefore the mess (and they are messy) falls on the floor not on the wall, *tip* must be out of the sun and if you buy commercially made reduce the entrances significantly (Sparrows who said anything about Sparrows!!)
Tony Clark