Does anyone have any experience of what happens with
a 'low' cable break. One of the advantages of a 'heavy'
wire is that it assists in preventing the tendency
of the launch parachute to fly up or is the parachute
unecessary with synthetic line?

Using Dynema, replace the chutes as well. You will only need a tiny chute
to keep a little tension on the cable when winching it in after release. I
have seen clubs use GRP discs for that. We us small chutes (about 40 cm
dia). Do about 12000 launches a year with them, no problem, including
ropebreaks.