I messed up my C3 experiment can i retake?

For paper 3 of Edexcel Chem A/s level you have to do a few experiemnts aswell as the paper at the end of the year.

One of the experiments on finding out what 3 salts were- i messed up - i didnt get the salts and most of the 'inferences i wrote' for the 15 marks were totally wrong.
Am i legally allowed to repeat it? I.e can i force my school to let me do it again.

Also roughly what percentage of the paper would it be worth?

thanks!!

Are these experiments done throughout the year at your school, internally asseseed and the maks sent off to Edexcel in the summer?

yes....