Greedy, good luck on getting the interview. All I can suggest to you is that you look hard at the job description which came with the job advert, and emphasise at the interview those parts of the JD which you are good at and where you have experience. Generally a band 3 job is more autonomous than a band 2 - so if you would be working more under your own initiative delegated within the limits of your JD, whereas on a band 2 you would be working under instructions from someone else. It's more responsible.
You can try to show also that you know something about the department where you would be working, what sort of work they undertake there. You don't have to know all the answers for the interview, you can say that it's a new area but you are willing to learn if you are lucky enough to get the job.
Why such short notice? Is it an internal vacancy?