I emailed Xero! They replied: "right now it's currently not possible to add a running balance column to the Customer Statements. However this is a request that's already been raised with us and you can vote for this request."

Helpful... they also sent me a link to the Microsoft Support website

Thanks very much for the info and links. I am going to delve deeper over the weekend.

I can confirm that the opening balance will always be zero though. Also that you are correct, «Balance» is the difference between «Amount» and «PaidAmount» for each individual invoice or row. Not bad for a non-accountant