Preheat the oven to fan 160C/ conventional 180C/gas 4. Grease and base line a 20cm round cake tin, about 5cm deep. Sift the flour, cinnamon and caster sugar into a bowl. Make a well in the centre and add the egg, milk and melted butter, then combine with a wooden spoon or electric whisk. Beat well to make a thick, smooth mixture. Spoon into the tin and spread evenly.
Remove the stalks and stones from the cherries, using a pitter if you have one to keep the fruit whole. Scatter the cherries over the mixture and gently press them in.
To make the topping, tip all the ingredients into a bowl. Rub in the butter with your fingers to make a crumb-like mixture, then work the mixture until it comes together in pea-sized pieces. Scatter the topping over the cherries.
Bake for 30-35 minutes until a skewer pushed into the centre comes out clean. Leave in the tin until cool enough to handle, then run a knife round the edge, remove the cake from the tin and leave on a wire rack until completely cold.
For a picnic, make the cake up to 24 hours ahead and return it to the clean tin when cold. Dust liberally with icing sugar, then wrap in cling film or foil.