Greetings from Sheffield in Yorkshire.
I'm glad to hear you have finally got a visitor to your nest box, over here we tend to clean our bird boxes from the end of September when the birds have stopped nesting.
People that have cameras in their boxes have seen Blue Tits and Wrens sleeping in them over winter without any straw in them, but once cleaned out there is nothing to stop the birds putting bedding in themselves for winter time.
Hello Tiziana, welcome from me up at the very top of Scotland, in Caithness. Lovely that you have the sparrows nesting, you will have fun following the goings on.
Thanks Alan. The end of September here will be too early I am afraid. It is still very hot in fact and broods could be delayed somehow. Beginning of November will be the right time, and I will follow your advice. If the couple will decide to sleep there during winter, it means that they will take some blanket in by themselves. :-)