Indemnity Charges

I am in the process of completing a claim for my mother's life insurance and am now needing the indemnity form to be signed and witnessed. My sister has already done this with her bank and had no problems, but on going to my bank (the same bank just a different branch) I was told there will be a charge based on the sum of the policy (it's a small claim under £40,000) and my sister shouldn't have had her signature witnessed without charge.
The bank isn't willing to go any further until I have proof of the sum of the policy, which I can provide in the coming days. I was just in need of any advice as to how these matters are usually dealt with.
Should I be facing a charge, if so is there a ballpark figure that can be thrown at me? Will my sister need her signature redone or will the bank need to contact her branch and have a fee paid before the forms can be completed?

Any information would be much appreciated.