Yes, it's the HK blue rubber. Outside diameter 8mm, inside diameter 6 mm, I think.
I know that it's not stretching much, 'cos I don't walk too far before the tension is quite high ... this doesn't accord with the tension that is required to stretch the rubber "on the bench". The rubber stretches many times its own length "on the bench".
I've got 100 feet of rubber and 400 feet of line. (55lb monofilament)
I've only tried it with a 2 meter glider ... generic "nothing-model".
I'm probably only walking back 50 or 60 feet before the tension is all I can manage with a one-handed grip on the plane. The original setup that I used, was much more compact and most of the rubber/line could be laid out on our cropped runway, if necessary.
I'm going to have someone tension the rubber separately, whilst I walk back with the glider. To be fair, the grass is very long on our field; it's rough pasture that is grazed by a few cattle. And, as I say, the glider-and-pilot end up slightly below the crest of the hill, when the line is tensioned, so the pull is not direct. Oh, yes, and the grass was wet when I tried out the new hi-start.
I'm not complaining about the HK' rubber, that stretches pretty effectively. No, I'm sure that the problem lies in the way that I'm trying to use it.