You will need 1 x 23cm / 9-inch springform cake tin and 1 x roasting tin, for water-bath. Preheat the oven to 170°C/150°C fan/gas mark 3/325°F and put a full kettle on to boil. Wrap the outside (underneath and sides), of your springform tin with a double layer of ovenproof clingfilm, then cover that thoroughly with a double layer of foil. The idea is to give a good waterproof casing to protect the cheesecake as it bakes in its water-bath later.Process the digestive biscuits with the butter until you have a sandy rubble that is beginning to clump, and press into the bottom of the prepared tin. Sit this in the fridge and clean out the processor, making sure you have no crumbage left whatsoever.Mash the bananas well with a fork, add the lemon juice, and set aside for a moment.Process the cream cheese until smooth, then add the eggs and sugar. Last of all, add the mashed banana and lemon juice, processing until you have a smooth mixture.Take out of the fridge the well-wrapped, biscuit-lined springform tin, sit it in the centre of a roasting tin and pour the cheesecake filling into your springform.Now put the roasting tin and springform into the oven and pour the recently boiled water into the roasting tin so that it comes halfway up the springform. Cook for 1 hour and 10 minutes, checking after 1 hour. The very centre of the cheesecake should still have a hint of a wobble, but should seem set on top.Remove from the oven and, still wearing your oven gloves, take the springform out of the water-bath and place on a cooling rack. Gently and carefully peel away the outside layers of clingfilm and foil, and let the cheesecake continue cooling on the rack.Put the cheesecake into the fridge but don't cover it till it's fully chilled, then leave overnight - and remember to remove it from the fridge about ½ hour before you want to eat it.To make the sauce, melt the butter with the golden syrup and sugar in a saucepan over a gentle heat until everything comes to a bubble, then let it bubble, keeping an eye on it, for 1-2 minutes. It will be a foamy, amber mixture like liquid honeycomb. Then let it cool slightly before pouring into a small jug and leaving to cool further; it will thicken as it cools.To help unmould the cheesecake, work a spatula around the top edge, before unspringing from the tin, then put it on a serving plate, preferably one with a lip. Whisk the toffee sauce in the jug and drizzle some over the cheesecake, leaving the rest for people to add greedily as they eat.