Fortunately, this can be solved by simply going to the website.
Unfortunately, the website is not very well designed, so it took a while to find it!
However, looking at the section about entering your own meter reading in, it is clear that this is indeed a "credit", meaning "they owe you money".
Notice how the costs break down. They estimated an energy usage (cost equivalent) of £104.09, which resulted in a "bill" of £29.77 (credit). Then the customer entered a meter reading, which resulted in an actual energy usage (cost equivalent) of £142.45. Since it was £38.36 higher, it went from a credit to a debit of £8.59.
Were £29.77 (Credit) to mean money was owed to SSE, they would owe a bit over £68 instead given the higher energy charges.
You can see this help page to inquire about getting a refund, or simply allow this to carry over to your next bill. Or - consider doing a self-entered meter reading, if one hasn't been done recently, to make sure that any actual excessive usage comes out of your credit (rather than being a shock at one time).