Preheat the oven to gas 150 (fan) 160 (regular)
Grease the tin well. I use melted butter and paint it on with a pastry brush and when that has cooled a dusting of flour which helps to find any places on the tin I've missed to grease.
In the mixer cream together the butter and sugar until fluffy.
Add the eggs one at a time, mixing in well each time. The mixture will get lighter.
In a separate bowl measure out and sieve the flour, bicarbonate of soda and salt.
Add the smoothie and mushed up banana into the butter/sugar/egg mixture and mix well.
Sift in the flour in two halves. Mix this slowly until all the flour has disappeared.
Separate the mix into two bowls roughly 50% in each.
Into one of the mixes add a little red food colouring and mix in well. You can make the red as dark as you like. I did this so I could marble the cake and make it light and dark to look like the red strawberries and creamy banana.
Put heaped tablespoons of the different coloured mixes into the tin overlapping them and mixing them to make a marbled cake.
Bake in the centre of the oven for about 1 hour 15 minutes or until a skewer comes out clean. When the cake pulls away from the edge of the tin the cake is ready.
Allow the cake to cool in the tin before turning out.
Sprinkle with icing sugar and enjoy.