Preheat the oven to gas mark 4 or 160 degrees fan.
Chop the walnut halves into small pieces, then add to a large bowl, with the the oats, peanut butter and maple syrup. Mix well.
Melt the butter and stir into the mixture.
Press into a lined loaf tin and bake for 25-30 minutes, or until slightly golden.
Leave to cool and slice into approximately 8 bars. Store in an airtight container.