Thanks for the comments.
The noise I've experienced is nothing to do with the car being left overnight in wet conditions. This always clears quickly after initial braking.
It is most likely, as has been suggested, something getting caught between the disc and shield. I've not experienced this with another make of car and I've owned a lot of cars since 1964. Also, it always seems to happen when going downhill.
As this is the first Jazz I've owned, I've been wondering whether it's due to some deficiency in the design of the braking system. It's a horrible noise and you expect to see some disc scoring, but so far, I'm pleased to say, nothing seen! Thankfully it's infrequent..