Ivor, I confess I don't fully understand your last two posts. I can imagine your steel box section under the keel, but have you left it hollow inside while sealing the ends? If so, you are negating some of the benefit, as that trapped air is buoyant and will act against the ballasting effect of the steel. At the very least you should allow water into the steel tube, and ideally the piece you attach should be solid. Or fill the tube with something denser than water. I also imagine you are fitting fairing pieces at each end of the tube to blend into the keel line rather than having a square blanked-off end facing the flow of water?
As for replacing the servo lines, are the old ones still in place? If so, carefully superglue the end of the new line to the end of the old, and use the old line to pull the new one through. Just overlap the lines by a small amount and keep the ends well glued to prevent fraying and snagging, and take it gently. If this fails, or the old lines aren't in place anymore, I suggest some thin flexible wire such as fishing trace wire, which you should be able to push through the tubes and around the bends, then attach the new lines as before and pull through.
Hope that helps a bit. If you still need help, can I suggest posting a couple of pictures of the offending parts as it makes it much easier to diagnose the problems.
Happy modelling!
Greg