Inverted sugar syrup is sucrose-based syrup treated with the glycoside hydrolase enzyme invertase or an acid, which splits each sucrose molecule into one glucose and one fructose molecule.
It seems that if you do it with just some citric acid, heating it on the stove, you end up with partially inverted sugar (ie - part sucrose, part glucose+fructose), but not sure what percentage gets inverted. They say golden syrup is around 44% sucrose and 56% invert, but no mention of whether you can reach those kind of percentages with the simple citric acid+heat method.
From the PDF you linked, they say that 100g of invertase will fully convert 100kg sucrose syrup (adjusted to pH 4.5 with citric acid) in 12 hours at 50 degrees C. If you deactivated the enzyme after conversion (by boiling, I would assume - much like boiling deactivates the enzymes active during the mashing process when brewing) you should be able to then add plain sugar (sucrose) back into it in the right proportions to reach the percentages listed above for golden syrup. Of course, if you wanted it to be golden, you might have to go through a heating process to caramelize it to the right point. If you can actually buy invertase in small quantities (as you'd only need 1g per kg of sugar), it shouldn't be terribly hard to do, though it'd be nice if it didn't take 12 hours. They say that if you cut the amount of invertase in half, the time doubles, suggesting it's a semi-linear relation - in which case you could theoretically use quite a bit more enzyme and speed things up accordingly.