We have had House Martins here regulary for the past 5 years Neil and we have modern plastic soffits etc which they don't seem to mind.We've never had more than 1 nest built on the house though.
My husband is planning on building a few Swift boxes over the winter which we'll site around the corner away from the Martins.
Apparently Swift boxes should have a postbox shaped entrance hole leading onto a ledge,he has found a design that incorporates a Bat box at the back of it as we have Pipistrelles here too.
Some info here;
http://www.londons-swifts.org.uk/Nes...Attraction.htm