Increase of instalment based on the incorrect info

Earlier this month i had an incident where someone smashed the back of my car and ran away. I contacted first for women for claim and it took them more than two weeks to give me a positive response. Last week Friday they called me and told me that the claim was approved but the premuim will go up. I ask for the reason why and they told me that the premium was wrongly calculated so they have to increase it by approximately R 340. I believe the mistake happened on their side and i disclosed everything to them. I am not willing to pay the increased premium as i have a monthly budget hence i did a quotation with them and was happy with the previous premium.