Rope advice

Excel pro is polyester core and wont make a good splice for your continuous lines.

Ideally you want English Braids / Marlow Continuous control / excel control. These have a dyneema or technora outer and therefore when you splice them you dont have any diameter change.

Aleternatively there are any number of poly cover / dyneema core lines about the can also be continuously spliced, however you will have to taper the outer to allow acess to splice the core. This will mean you do have a diameter change in your lines. Not a major issue, i tended to jut open both cleats between races and run the line round so that the splices were in the takeoffs instead of near the cleats or in a loaded position,

This is slightly less relevant if you dont have cont. lines, but the 600 ideally needs them! and if your re-roping now is the time to put them in..
If oyu want a control line setup diagram for the 600 give me a shout, as i made a somewhat crude one a while back.

Thanks a mil for that guys - that's very helpful indeed. The thing is, I've never spliced a line before. Is it something I can pick up from YouTube tutorials, or is it something you really only master with years of practice? My control lines are all a continuous set up. The kicker rope needs immediate attention. Do I need to do any splicing to replace that?

Have a look at www.dinghy-rope.co.uk for rope, 

No they are not. They are owned by the same person, but the two companies are run in direct competition.

Think 4mm is smallest fid Jamie, you'll need either one of the long needle type or a wire loop type. I prefer the needle type for 3 &4 mm but then a wire for sizes like 2.5 down