Advice regarding House Martin nest

I hope someone might be able to give me some advice regarding artificial nest boxes for House Martins. We have House Martins nest under the eaves of our house every year (been here since 1988). We need to replace our old soffits and facias before they come back this year. I have read that the mud does not stick to UPVC and their nests fall away from the wall. I don't want to lose these birds and I know they need all the help they can get so I'm thinking about getting a nest 'box' put up. Has anyone used them and are they successful? Thanks Sue.