Preheat the oven to 170°C/325°F/gas mark 3 and lightly grease a 9 inch springform or push pan.
Place the Digestive biscuits, pecans and sugar in a food processor and blitz until it forms fine crumbs. Add the melted butter and blend again until it resembles wet sand. Tip the mixture out into the prepared tin, spread it level and pack it down firmly.
Whisk together the cream cheese, cornflour, sour cream, sugars and vanilla until smooth then whisk in the eggs one at a time.
Put the kettle on. Wrap the outside of the tin in a double layer of clingfilm then a double layer of wide tinfoil, making sure that there are no gaps or splits; place the tin in a large roasting tray.
Remove the cheesecake from the water bath, return it to the switched off oven, close the door and leave it in there for at least an hour (letting it cool down slowly like this helps to prevent it from cracking). Allow the cheesecake to cool until it is almost at room temperature then carefully run a knife around the edge to loosen it from the tin before covering and refrigerating overnight.
Place the sugar and water in a heavy-based saucepan and place over a medium heat. Cook, swirling the pan (don't stir), until the sugar has melted. Continue to cook until it turns amber then remove from the heat and carefully pour in the cream mixture (it will splutter). Return the pan to a low heat and stir until the caramel is smooth. Stir in the pecans then set aside for the caramel to cool a little.
Carefully turn the cheesecake out onto a serving platter, pour over the pecan caramel and serve. Both the cheesecake and pecan caramel can be made the day beforehand and stored in the fridge but don't top the cheesecake with the caramel until shortly before serving. If the caramel gets too thick to pour then simply warm it up over a low heat until it melts. Keep the cheesecake refrigerated.