those look like soil millipedes. They do eat decaying plant matter in the soil. If you have small seedlings you might want to keep them away to prevent accidental feeding from them. Think of them as composting worms with a lot more legs

Botanybob, That photo is the closest I've seen. The ones in my garden are slimmer, but maybe just a younger version of the photo. I checked out wireworms, and the things in my garden had more legs than the photos I saw of the wireworms, so I don't think it's those (happily, after researching wireworms!). Guess they must be millipedes.

It looks like a picture winged fly. I see them around, don't think they cause any trouble.

Floral you have a good eye for plants not native to the British Isles! I saw a funny picture on the web years ago: at some botanical garden in the UK is a Toxicodendron sp. in a wrought iron cage. If you cut junk or invasive plants at the base, you ought to treat them to stay dead or else you will be back to cut them again, and again, and again. My preferred method is a commercial glyphosate diluted to 25% and mixed with a bit of surfactant and blue dye for marking purposes. Bring a 3/8" inch drill, dental irrigation syringe and herbicide solution to "job site". Drill hole in cut stems and fill with the dyed "roundup" solution - aiming for at least 1.5" deep. Guaranteed to work as long as there's no rain to wash it out in 24h. Dose roughly as follows: anything < 1 inch, 1 hole, 1-4 inches diameter, 1-1.5 holes per inch diameter around the edge, > 4 inches diameter, a hole every 2-3 inches along the outer edge. (hopefully you didn't let the weed tree get that big!)