I have used Marlow Ropes Excel Vectran 3 mm diameter.
Photo shows it threaded through the Cunningham upper double block; no problerm threading.
I bought sufficient for two long tails that make up a doubled working part the same length as the original RS tapered line. And tie the ends together in a double figure of eight knot. In several years of using: on two Aeros with all rigs this has not jumped out the goose-neck.
The Vectran is obtainable form Pinnel & Bax, cut to the metre. On their web site, www.pinbax.com it is said to be flexible through tight radii.
When I need to replace it I use 4mm dynemma (light, strong, chaffe resistant and minimal stretch) It may need a little coaxing/pushing/pulling through the cunningham block that it ties to.
I need to replace the line that goes from Cunningham's blocks through the tack eye and attaches to the mast. Not sure what type of rope to use or if there is a pre-made line available? Any help appreciated thanks Steve