My next comments may sound any of the following: stupid/ untutored/niaive.. Any could be valid!
I’ve gone onto the Uswitch website and ticked the box for having Economy 7 at my (shown) address (this leads me to believe my MPAN no is now correct on thenational database). If I tick one more box it seems it will let me change to Scottish Power Economy 7 Rates – fixed until December 2017. 
Andrew, I think you seem to be the one to give me the answer…. Is this stupid? I simply do not understand if switches need to be flicked/depressed somewhere to supply me with 10 hours cheap rate, or would my installed metre be the “regulator” to ensure I have 10, not 7, hours cheap electricity daily. To clarify, is this action feasible without speaking to SP? Might/will I achieve the exchange to Scottish Power by this online process ? And, NB, the online December rates are below the verbal version! Thanks to you all for your patience.