SP Have now reduced my bills for now. Thanks for all your replies everyone. Unless you have access online, you would never get in touch with SP anymore via the phone. I think they need more phone operatives.
I have the same issue in mid May 2019 I noticed that my Scottish Power account was some £1800 in credit and the web site informed me that I could reclaim this: I couldn't. I called and was informed that the money could not be reclamed without a meter reading. The meter reader arrived a week later. I called Scottish Power on the 12th July and was informed that the meter readings were only received from the meter reader on the 1st July and it can take up to four weeks for the system to update. Apparently, I'll be able to reclaim the money after the 1st Aug. Just three and a half months after noticing the problem. The lesson is clear, monitor your account frequently and act quickly if it starts going wrong.