They never made Ron Hill running legs long and thin enough for James, but in the early 90s at a rowing regatta we found something called "4runners" that did fit, and he bought two pairs, in blue and black, and they've been used for running and cycling ever since. I spotted some rather similar looking/feeling fabric in Yuzawaya in Kamata. Just this year James decided that his blue pair were past it so he wore them on a trip to Kamata and we decided the fabric was near enough to give it a whirl.
I took the pattern off the old pair, using the voile left over from making the beer-bag. It is partly transparent, supple but not stretchy and I was able to make a pattern without cutting up the old pair. The trick I missed was that, although the widthways stretch was similar to the originals, the lengthways stretch of the new fabric was quite a bit less than the 4runners fabric. I'd already added an extra inch to the legs for luck, but it really wasn't enough, and I ended up having to add 5cm to the under-foot straps. I actually put the cord and toggle from the old leggings back in the new pair. I didn't bother with the piping on the sides for this pair, which were really a test fit. He is wearing them for a while before I embark on another pair. Apparently I may need to lengthen the rear crotch too next time.