After a lot of grief, I got the regulator involved (a process which brings with it its own delays). SP agreed to cancel the erroneous debt and provide me with a derisory amount of money as compensation. I told the regulators that I found the compensation payment insulting, and wanted a full explanation and apology for what had happened. As a result I got f*** all money and a one line letter from SP saying (and I'm quoting more-or-less verbatim here) "we apologise for an error on our part".
At the risk of repeating myself, a complete shower of c***s