Dont forget there'll probably be a compulsory excess on your policy as well which you would add the voluntary excess to to find the total excess figure.
That caught me out on my policy
£300 voluntary excess but hidden in the small print was a £400 compulsory excess. Ouch!
If the fault is 100% for the third party then I believe that you don't pay the excess
I believe that you pay the excess to get the car repaired if the fault is disputed then you receive a refund for the excess paid when the fault is determined to be the third party and your insurer recovers their costs
You shouldn't have to pay any excess. It's why everyone has to have at least TPFT. Means his insurance company will pay you all the money for your repair/replace, while he has to pay his excess to the insurance company.