Golden syrup, long popular in Britain, is becoming more broadly available in this country. Lyle's Golden Syrup is the most common brand, and we have seen it in supermarkets in some pretty out-of-the-way places. It is also available in specialty stores and online.
Golden syrup, like molasses, is a product ofthe process of refining sugar. It is simply sugar cane juice that has been boiled down. It has the consistency of corn syrup, but a golden color and a taste different from either light or dark corn syrup, and also substantially different from its cousin, molasses.
Your treacle tarts are unlikely to taste as good as you remember without the use of honest-to-goodness golden syrup, but if you must, you can try substituting it with 2 parts light corn syrup and 1 part molasses or equal parts of honey and light corn syrup.