CRE Loaded 6.4 - Check payment method by default

I need to have my CRE 6.4 B2B installation tweaked a bit to automatically select the top payment method on [login to view URL] If the user continues to the next checkout page, then returns back to the payment page, I'd like it to retain the payment method they have selected. I'd like to know which files and/or database changes are required to implement this change again if needed as well. And this is unrelated, but while you're on that page, if you can fix the 'redeem code voucher' button's graphic, which currently seems to wrap around under itself as if the second/third part of the image is on the line below the first part(s).