I haven't made treacle toffee in donkeys years, so I can't give you a recipe I've tried and tested myself, but here's one from the Foody to keep you going !
http://thefoody.com/sweets/treacletoffee.html
You might need to buy a sugar thermometer but if I recall correctly, the mix is at the right temperature when a little bit dropped into a saucer of cold water solidifies straight away.
This second recipe describes the process better !
http://www.greenchronicle.com/recipes/treacle_ toffee.htm