I think this is a velvet mite. It was crawling on the underside of a leaf that was covered in a rust-like fungus, but whether it was feeding on the fungus, or if that was just coincidental, I don’t know. It was smallish (a few mm, 3-4 perhaps) but quite active. The guide shows a couple of different velvet mites that appear similar, but despite looking at the images for some time, and I cannot figure out how the two types are distinguished one from the other. Thanks.