I'm 3rd Party, hit pedestrian, how do I pay the excess, no damage to my car/no claim

Hi There.

I have hit a pedestrian going at 10 miles an hour. Police called and they said no charges will be brought and they say its a 50%/50% fault. I am third party and am not going to claim on my insurance, as only a small dent on bonnet which will cost me £45 to sort. However I guess the guy I hit is going to make a meal of it and claim against my insurance for all costs (he was fine and probably didn't even get a bruise). What I need to know is how will I pay the £400 excess I have although I am not claiming myself? Do I have to pay it directly to my insurance company as one lump sum?

Does anyone know if I still have to pay even if I am not making a claim.

Thanks