I've been brewing with black treacle for some time now with my stouts, porters and other (UK) ales. There are many folks on the forums that will swear that it's the same as black strap molasses, but I wholeheartedly disagree. It has a much more complex and richer flavor than that. I can't seem to find the source, but someone once told me that it isn't just cane sugar (like molasses) but also contains beet sugar - dunno. What I do know is that there *is* no substitute in my brewery. I love the stuff!
I mention this because Lyle's Black Treacle can be hard to find. (Their Golden Syrup - also very delicious - is much more readily available.) When I lived in Cleveland, the only store that carried it was about a half hour's drive and it was about $5 a can. Well, Food of All Nations (
http://www.foodsofallnations.com/) carries it and best yet, it's just above $4 a can.
Cheers-