It's a bank. I had to email mine to find out what I could overpay. Turns out I'd completely misunderstood my allowances and probably could've paid off my mortgage years ago. Ok, I'd probably have to have given up the coke and hookers but you know, it's the principal of the matter.
It says "ERC on the amount repaid"
Presumably it's the amount over the 10% allowance.