Combine brown sugar, honey and peanut butter in a microwavable bowl. Melt the peanut butter mixture in a microwave in 30 second intervals, stirring after each melting, for 1 to 2 minutes (depending on your microwave). Stir in Weet-Bix and raisins.
Press into an 20cm square tin sprayed with non-stick spray. Cool and cut into bars.