Swallows nests

Wondered if anyone could tell me - I know swallows return to the same nesting site year after year, but do they re-use the same est, or start afresh each year?

I ask because when we moved in to our new house, the garage was full of old swallow's nests. As we re-sited our boiler and washer in there, we hard-heartedly blocked up all the entrance holes to avoid everything getting pooped on. We've had a nest this year on a little shelf put on the outside of the shed to encourage the swallows. The youngsters have now flown (letting me finally patch up the leaky roof!) and I'm wondering whether I should leave the nest in place for next year, or clear it away?

Anthony