Swallows nests

Hi there, I have a question about swallows nests which someone may know the answer to.
I know swallows return to the same nest site year after year, but do they re-use the old nests or build afresh each year?
We had several nests in our garage when we moved to our new house, but hard-heartedly boarded up the openings and kept the door shut, as we had re-sited the boiler, washer etc. there and didn't want everything pooped on. We have had a nest on the outside of the garden shed this year, using a little shlef installed to encourage them. The babies have just flown the nest (allowing me to finally get on and mend the leaking roof!), and we are wondering if we should clear away the old nest before next Spring or leave it for them to return to?
Answers on a postcard please.... )

Anthony

 

Just realised I should have posted this in the Wildlife forum - sorry! Admins feel free to move it!

A