For those who might try:
Very fine slice a dozen big limes. In plastic, glass or porcelain, soak overnight in 7C water.
Bring to a boil then simmer 45-60 mins to cook the lime peel.
Add 1kg sugar (original recipe says 2 kg) stir to dissolve and cook again, with gentle stirring 45-60 mins.
The whole batch may turn a bilious, nauseating yellow. Stir in 1 drop blue food coloring. Fairly realistic.
This should gel nicely. Into jars and process. I preheat my jars to 250F and load on a tray (if one breaks).
I've boiled the lids, rings are ready. I got nearly 10 x 250ml this time. Depends on evaporation in cooking.