This is, I take it, in a lit field? If it were my [science] field, I'd do it just like a 15 minute conference presentation. State of the problem, say why it's important, give info on method, give results, conclude, answer questions. (Then, of course, get a beer.) Hopefully, that's adaptable to your field.
This wouldn't work here. You need to place your work in the broader context of the field - why is it interesting and important (i.e. what makes it "internationally excellent" or "world leading"). You need to think about how your research would help the department achieve its REF objectives and attract funding from agencies such as AHRC.
(I suggest you Google the acronyms, you really need to know this stuff.)