Sorry @An Ankoù I had never even heard of invert u til.this thread so don't think I can off much advice other than what I did.
This is the sugar that I used
I used just under 500g and 1 pint of boiled water from the kettle but added a bit of cold when the temperature looked like it might go over when I was doing the initial heating up (a tip that I read somewhere but can't remember where that was), then I just turned the heat down and let it simmer away. The only bits that started to turn a bit solid were when they cooled, either on the side of the pan or on the spatula that I was stirring with.
This is the sugar that I used
I used just under 500g and 1 pint of boiled water from the kettle but added a bit of cold when the temperature looked like it might go over when I was doing the initial heating up (a tip that I read somewhere but can't remember where that was), then I just turned the heat down and let it simmer away. The only bits that started to turn a bit solid were when they cooled, either on the side of the pan or on the spatula that I was stirring with.