I'm sure you've considered this Peter, but an alternative to trying to prolong bites/building slack in is to do the opposite. There are lots of decent dace in my local river, and in some circs you can't float fish for them. Legering with any conventional rigs will make you pull your hair out over fast bites you just can't hit. The answer is a miniature bolt rig - a little feeder running on a short length of twizzled line between a stop below and a knot that provides a "bump" on the line above. The dace hook themselves against the bump, but as it's only a bump, not a stop, it will pass through the swivel on the feeder if you hook a bigger fish and avoid sit's where the bigger fish snags the feeder in weed or whatever and cracks you off. I'm not generally a user of self-hooking rigs, but if you set this up right you go from hitting about 40% of bites to coming back with a fish nearly every time.
Professor Scotthorne, 5x world champion, shows how to tie it here. The rig bit starts around 4.30
Professor Scotthorne, 5x world champion, shows how to tie it here. The rig bit starts around 4.30