Here is the final bird nestbox repaired and renumbered. I waited until I had cut and painted all the number tags before making any entrance guards from the HDPE sheet; just in case there wasn't any spare material.
Regarding nesting material. Blue tits in my garden have been using a bright yellow acrylic wool type of fabric in their nests for several years but I have no idea where they have been finding it. Looks a bit like those really fluffy pullovers!
Last year they used the same thing but a bright green colour.
We don't get too much problems here from fly tipping. There is a nearby recycling centre but they have begun to get fussy about what they will accept so maybe in the future people will have no option except ditching unwanted items.
The local authority will pick up any dumped items, but they are becoming more reluctant about doing this. They will also collect any large household items but there is a charge so many people cannot afford to call them.
This problem always seems to be worse near cities; we are an area of small towns and villages