M&S Energy is a website that is owned and operated by SSE Energy Supply Limited (trading as Scottish Hydro, Swalec and Southern Electric) but which uses the Marks & Spencer brand for the purposes of supplying energy-related products, services and information.
However, it's always definitely worth finding out what M&S tariffs are currently available as they could well be cheaper from a unit/price perspective than SSE's own tariffs. And dependent on the promotions available at the time, you may even be offered the reward of Marks and Spencer voucher for changing to M&S Energy.
Non-Premium Rate Phone Numbers include:
0800 975 4754 Customer services
200 Dunkeld Road