These are three similar recipes from Aust. cookbooks. I have seen a recipe that has cream of tartar in it but it wasn't for plain old toffee.

Toffees - Aust Women's Weekly
3 c sugar
1 c water
1/4 c malt vinegar eg Cornwells
Combine over low heat in a saucepan, stirring until sugar is dissolved. Increase heat, boil rapidly, NOT stirring until 154oC or Hard Crack stage [approx 15 mins]. Remove from heat, allow bubble to subside then pour into patty case. Decorate as desired.

Toffees - Lady Flo Bjelke-Petersen
2 c sugar
3/4 c water
4 ts vinegar
Combine over low heat in a saucepan, stirring until sugar is dissolved. Increase heat, boil rapidly, NOT stirring until 154oC or Hard Crack stage [approx 15 mins]. Remove from heat, allow bubble to subside then pour into patty case. Decorate as desired.

Toffees - Aust Family Circle
2 c sugar
1 c water
2 ts vinegar
2 ts honey
15 g butter 1/2 oz
Combine over low heat in a saucepan, stirring until sugar is dissolved. Increase heat, boil rapidly, NOT stirring until 154oC or Hard Crack stage [approx 12 mins]. Remove from heat, allow bubble to subside then pour into patty case. Decorate as desired.