Hi Oneeyedvic. You can't be expected to know about the differences you will make to an organisation that you do work for (as you might not know what they currently already do!), so the best aspect is to focus on your skills themselves. The business will be looking to see someone who is dynamic, committed and keen to implement realisable (and effective) changes within their current structure, therefore dannydingbat's advice is really good. Think about structuring your presentation focussed on say 4 key points, and order in a way which develops naturally to a conclusion. If the interviewers have a number of people to interview, then they will be looking for something to remember, so ensure that your slides are attractive, well laid out and checked and double-checked for spelling, grammar etc. As they will be looking for someone whos attention to detail is strong, ensure that they will be impressed with anything put before them.
Overall, ensure that your message is focussed on the strong skills you will bring, but be sure to include the fact that you believe in continuously learning too, and that as an employee of their company you will be keen to develop the processes that work within the business, and drive change in the areas which are not so strong.
Best of luck.