A simple stop knot. I first used it to stop sliding floats.
take about 4” of the cotton (or thin nylon or thin pole elastic) hold it against the mainline, make a loop with the cotton and then pass the end 4 times thru the loop. Pull gently to tighten, then position where you want it and properly tighten.
cut loose ends to suit and add a dab of nail varnish as a fixed point - for reference.
Extra long range feeder fishing. In fact a boilie boat maybe more suitable. Nicky is in North Yorkshire, Southfield reservoir is in East Riding of Yorkshire and I’m in South Yorkshire Mark. You could throw a travel blanket over all three of us. I’ve just dropped unlucky. ???