Line your muffin tin with large cupcake cases. Set the oven to 190c / 375f / Gas mark 5.
Sift the flour, salt and bicarbonate of soda together in a mixing bowl.
Beat together the baking margarine and sugar in a bowl until creamed. Beat in the eggs.
Mash the bananas for the cupcake mix in a small bowl until mushy. Add the lemon juice. Stir this banana mixture into the creamed fat and sugar. Work in the dry ingredients.
Spoon the mixture evenly into the 12 paper cases. You should have enough mix for a generous dozen.
Bake for 16-20 minutes, depending on your oven. I tend to do mine for 12-14 minutes then turn them around 180 degrees so they brown evenly. Allow to cool in the tray for ten minutes then transfer to a wire rack to cool fully.
For the cream cheese icing, give the banana, lemon juice, milk and cream cheese icing a quick whoosh in the hand blender so it is fully broken up but not totally liquid. Transfer the gloop to a medium size mixing bowl and sift in the icing sugar 100g at a time. You want the icing to leave a trail on the surface when you drizzle with a spoon so by all means add 100g more than I've said if you think it's still too runny. Pipe the icing onto the cool cupcakes using a disposable bag with about 1.5cm snipped off the end. I did my icing in a circle so that you could still see cake around the edge but it's up to you really. (You could paste it on with a small palette knife if you don't want to fiddle with piping bags.)
For the toffee sauce, place the butter, sugars and golden syrup in a small saucepan and allow everything to dissolve over a low heat. Let it cook for 5 mins. I brought mine gently up to the boil and let it cool back down at bit. Pour in the cream and vanilla extract and stir in until the colour is even. Allow to cool in a mug - you should have around 300mls of sauce.
When the toffee sauce is cool, spoon it into a disposable piping bag with a tiny hole snipped off and drizzle it backwards and forwards across the banana cheesecake icing. If you are not eating them straightaway and presentation is important, I recommend you leave the iced cupcakes on the worktop (not in a tin) for 24 hours to allow the icing to set. If the icing still looks wet, your trails of toffee will merge slightly into the icing.
For the decoration, if using fresh banana, dip thin slices of banana in lemon or lime juice to stop them going brown. Turn them over so both sides are coated. Transfer to the centre of each cupcake using a pointy knife and you're done! Alternatively used dried banana chips.