i use daytime lights in a car all the time--the human eye sees light, then movement, then colour. lights can give oncoming drivers several extra seconds of reaction time.
so on a less than optimum day, i'm happy to have them on my bike--especially in the rain. it's less for drivers and more for other cyclists, as i'm often on the paths and i notice how many of us have our heads down in the rain.
"My usual lights have a day mode (bright double flash every so often) - no idea if they help but I’d like to think they might be useful at catching people’s eye at roundabouts/junctions"
i think i've seen these, and they're really weird. i noticed it the first time because i thought there was a bike heading towards me at dusk, and then it disappeared--on a path! suddenly, very near me, the light flashed again. i've seen these since, and find the gap between flashes counter-intuitive and difficult to guage how far away the light source is.