Remember your stages of sugar boiling and how they come out, if it's too sticky or soft it is probably down to too low a final temperature (wrong texture) or more likely too much golden syrup.
Be sparing with the golden syrup / glucose syrup, the sucrose will set in to hard sugars, but the glucose/inverted sugar syrup acts as a crystallisation inhibitor, vinegar can be used to similar effect (though does not enrich the flavour so).
It sets too, but I find too much can lead to weeping.
Also, I have heard that an overly humid environment can be a problem.
People used to say not to cook toffee on a humid day.
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In nigellas recipe she says to only let it boil for 3 minutes before adding bicarb.
When I make this I let it boil for 10 minutes before removing from the heat and adding bicarbonate and mine always turn out crispy.
cooking.stackexchange.com/users/22962/Zoe Rothwell
Correct sugar temperature is the main factor, bring it just up to the heat required, sugar will overshoot because of heat latency To improve bubble action add one teaspoon of plain white vinegar at begining of sugar heating processn
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Less golden syrup...
same happened to me, try 1 tablespoon instead of 4
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Did you make sure to add enough bicarbonate of soda for your sugar/syrup, AND to ensure you are not stirring the mixture once heat is applied?
That's the only thing I can think of that would cause it not to set solid.
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Most likely due to not letting the toffee come up to a high enough temperature.
Made it recently , and my candy thermometer said it was at 150 degrees C, but it turned out sticky.
When I made it using the old-fashioned "drop a bit of toffee in a glass of water to see if it is at hard crack stage", it was perfect.
3 minutes sounds far too short to bring it to a high enough temperature - mine took more like 10 minutes.
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I think often it can depend on the stove and whether it is electric or gas powered, but overall, as others have said, just cook it for longer.
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I made "honeycomb" (otherwise known as hokey-pokey, not real honeycomb) using this recipe:  http://www.nigella.com/recipes/view/hokey-pokey-36  It tastes delicious but it is supposed to be crunchy / cripsy (like the inside of a Cadburys  Crunchie bar) but it actually came out really sticky, like a soft toffee.  Is this likely to be due to overcooking, undercooking, something else? How can I make sure it comes out crunchier in future?
How to make "honeycomb" less sticky?
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<sugar><candy>
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Make sure that the sugar temperature is correct, you don't use too much golden syrup and your environment is not too humid. A teaspoon of plain white vinegar at the begining of the sugar heating process can help, or let it boil for 10  minutes before removing from the heat and adding bicarbonate. The sugar needs to be the right temperature and use less golden syrup. Try boiling for longer than in the recipe.
Make sure the sugar is the correct temperature. A humid environment can affect the result. You will need to boil it for longer than the recipe says. Use less golden syrup.