Mal,
The reason you want a small knot is so the ring can be raised above the hook to easily disengage them. I ended up buying a new halyard and tieing a buntline knot as suggested. I went sailing yesterday and I engaged and disengaged the ring on my first try. It was great! I think I am still going to replace the sheaves but I was very happy with how much easier it was by making the knot smaller and using a costlier halyard with less stretch. I put Mclube on my sail frequently and I've never had a problem with the luff pulling out of the track.
Matt