Formatting the "Other" choice field

I've built a form for making donations, with radio buttons for various amounts, and 'Enable "other" choice' selected so that there is a text input where the donor can enter a different value. (Form is here: http://mnds.the-graphics.net/fundraising/donations/)

The problem is that some donors type the £ sign in this field as well as the amount, which causes problems when the payment is processed. I would like to simply add the £ sign before the text input, but I can't see any way of doing this, as there's no place to add a label to that field.

Any suggestions would be appreciated!

Thanks for that suggestion David!

In the end, I added a custom validation on that field so it throws an error message if the £ sign is there, or if anything else weird is input. (See http://www.gravityhelp.com/documentation/page/Using_the_Gravity_Forms_%22gform_validation%22_Hook )