The reason why my email address doesn't work on the Scottish Power website is because the new flat is supplied by Southern Electric.
This is actually a Good Thing, as Southern Electric is a) significantly cheaper and b) have "no plans to increase electricity prices" (unlike Scottish Power who just stuck an extra 8.9% average increase on theirs).
Note to non-Brits: since privatisation there's no actual link between who you pay for electricity and who actually produces the electrons that you use. This is how in the old flat I paid Scottish Power for electricity, despite living on the other side of the country from Scotland. Of course, being Brits we've not completely got rid of all the pre-privatisation baggage, and so most suppliers have different prices depending on which former electricity board supplied your area.