I was a Customer Service Advisor so I'll write about what I know.
For hints as to how to conduct yourself or things to say, pay attention to the job description, check out their homepage, to see what's hot, and their CEO's blog.
Wear smart professional but no need to overdo it as they're pretty tolerant about looks in the work place: there's no dress code as such, just exercise your common sense (nothing offensive or unprofessional, logically). Performance matters more.
Show an open mind and willingness to learn, and that you're ready to handle what's thrown at you ("yes, I can do that" or "that's not a problem" are always winners).
If you pass this stage, you and other hopefuls will go on to Assessment day when, among other activities, you'll go through some written tests to assess your basic maths, language and other necessary skills. For the maths test, read the sheaf of printouts they gave you previously, it shows all you need to know. The test is short, nothing complex; don't worry if you can't finish it all in time, what matters is what you write in it, and HOW (be accurate, concise and to the point: given the volume of calls if you have to deal with daily, they need people who can think on their feet and juggle quite a few balls at the same time).
You'll know if you were successful or not within the half hour.
Company Values, History & competency question based answering.
Make sure you show passion for great customer service as well as safety. Look up SSE set values
Research the company and interviews are mainly competency based.
Look up SSE SET acronym and find out about the things which are important to Sse. This is on the website. Do your research, learn about the company and the different things they provide apart from gas and electric. You might be given a list of things to place in order of importance. Make sure you listen and show you are great at working as a team and as an individual. It's also important to prove you are good with meeting/beating targets.
You need to learn a lot of in depth information for the field that you are applying for .
Definitely read there home page and the blog from the CEO