1. Combine the water, caster sugar, and the corn syrup in a medium saucepan. Stir them together making sure no lumps of dry sugar remains. Brush down the insides of the pan with a little water, using pastry brush for any stray granules of sugar.
2. Cover the saucepan and place it over medium heat for 4 minutes.
3. After 4 minutes, remove the lid, increase the heat to high, and bring to a boil. Do not stir from this point on. Keep an eye on the pan. It will be very bubbly. When stray sugar crystals appear on the side of the pan, brush them down with a wet pastry brush.
4. As the sugar cooks, the bubbles will get larger. Insert a candy thermometer, and when the temperature reaches 300 degrees F, lower the heat to medium, which will slow the cooking.
5. Continue to cook the sugar until it reaches 330-340 degrees F. It will be dark brown. Don't let it get too dark. Remove the pot from the heat and let it sit for 1 minute, or until the bubbles subside.
6. Add the double cream to the caramel. It will bubble up vigorously, so be careful. Vigorously whisk.
7. Whisk in the remaining 2 tablespoon sugar, light sour cream or creme fraiche, lemon juice, and salt. This sauce is now ready to be served hot, warm or cooled.