So the sample has arrived in the post, as a manufacturer they have done something that is rather rare. They have improved a product so it is actually better. Usually "New improved" is another way of saying "messed with and made worse". The cord they have sent as a test sample is softer than the previous stuff, meaning it holds knots a lot better, and if you have a quick release knot, it undoes a lot easier than previously. I've tried tying a few knots and putting weight on the line, Evenk hitch, bowline, double figure of eight, and a line loc. All worked fine. So I will notify them tomorrow that it is good to post the cord to me!
I think we might be on the home straight now!
Thank you everyone for your patience.
Julia