Grandpa Beard's upside-down ginger cake

I don't know who Grandpa Beard is, but his cake is absolutely delicious...

125g plain flour, half tsp bicarbonate of soda, 1tsp ground cinnamon, 1tsp ground ginger, quarter tsp grated nutmeg, pinch of ground cloves, 1 egg, 125g light brown sugar, 90g black treacle, 125ml soured milk (to create soured milk, add a few drops of lemon juice to fresh milk), 50g melted butter, clotted cream, to serve
Topping: 50g unsalted butter, 100g light brown sugar, 400g pear halves, walnut halves, to decorate

Heat the oven to 180C/fan 160C/gas 4. To make the topping, gently melt the butter in a small saucepan, add the sugar and stir for 1-2 minutes. Smooth over the base of a 20cm cake tin with a removable base, 7cm deep. Arrange the pears on top, cut-side down, with a few walnuts around the outside, flat-side down.
To make the cake, sift the flour, bicarb and spices into a bowl, and add a quarter teaspoon of fine sea salt. Blend the egg, sugar, treacle, soured milk and butter in a separate large bowl, and fold in the flour mixture. Beat with a wooden spoon for 1 minute, and the pour the mixture over the fruit in the tin. Put on a baking tray and bake for 40-50 minutes, until a skewer inserted into the center comes out clean (try to skewer a patch with cake rather than a pear....like, doh!). Remove from the oven, run a knife around the edge and set aside to cool for about 10 minutes.
To serve, remove the collar and invert the cake onto a plate or cake stand. Enjoy warm or at room temperature (but avoid chilling), with a dollop of clotted cream on the side if you wish.