My clotted cream vanilla fudge just won't set. I am using a sugar thermometer. I'm melting my ingredients then bringing the mix to a temp of 115C and cooking at that temp for 20mins before increasing the heat. I've tried taking it to 121C and even last night to a temp of 130C! Then I'm beating with a hand held electric whisk until thick and matte. The fudge tastes delicious but it's still soft and squidgy.

Ingredients are:
275g caster sugar
100g golden syrup
227g tub clotted cream
1/4 teaspoon vanilla paste with seeds

What (else) can I do to make it set?