Preheat the oven to 180°C, gas mark 4. Grease and line a 25cm cake tin.
Beat 250g butter (put the rest in the fridge) with the caster sugar until light and fluffy. Add the cinnamon and honey.
Add the eggs one at a time, mixing well between each addition.
Carefully fold in the self-raising flour, walnuts and apples until just combined.
Pour the mixture into the prepared tin and smooth the top.
Put the plain flour and light brown sugar in a bowl and rub in the remaining cold butter, until the mixture resembles breadcrumbs. Sprinkle the crumble over the cake mixture and bake for 40–50 minutes, until a skewer inserted into the centre comes out clean.
Leave to cool for 10 minutes, then turn out onto a wire rack.
In a small saucepan, warm the toffee sauce but don't allow it to boil. Pour over the cooled cake just before serving.