Nuthatch in Blue Tit Box update

Yeah I know nothing about Nuthatches which are extreme rare in Bolton where I live but I've recently moved to the north of the town near the moors and have a small woodland area to the rear. I have no idea why they would block one hole up with mud from the inside although my guess is that it's so inbuilt in their genes that they have to use mud somewhere on the nest? They've even got used to me being near the nestbox although not disturbing it in anyway inc resisting the temptation to look inside it from the slide down front piece. I just felt doing that may damage the nest inside. However I will take a good look and take photos after they've fledged. Obviously I'll also clean it out thoroughly but can't see it being used again this year so late on.

I actually disagree totally with the RSPB on that one for a very obvious reason. As long as you know for certain that the box is empty and that all living chicks have fledged, then that is the time to remove the used nest and more often than not any remaining dead chicks. Not just for hygiene reasons but as has happened many times before the nestbox may be used again either for a 2nd brood or by another pair of birds who may have abandoned their previous nest etc. No small bird such as tits or nuthatches will ever use the same nesting material again so its pointless leaving it in your fledged nestbox. They want a totally fresh site and not the hassle and wasted energy consumption of having to take out all the old nesting material. So it baffles me why the RSPB give out the advice of leaving the nestbox cleaning til Autumn? Of course this is as long as you know for certain that the box is empty, which most ppl would if the box is in their garden. Maybe that's why mine regularly gets used twice a year! Be interesting to hear someone from RSPB comment on this please?