From my experience of running the postgraduate seminars at UCL, 20 minutes really isn't a long time to speak on a subject as complicated as mathematical research, so don't get bogged down in technical details and don't try to fit too much in. The PReSS talks were scheduled for an hour, but people often went over. A talk which takes 15 minutes to deliver by yourself will probably take nearly 20 to deliver properly to an audience because you will be stopping and starting to field questions and clarfiy issues.
You might be surprised (maybe not eh), but even understanding the statement of a theorem can be really tricky, let alone the proof of it; so try to think about your work on a more philosophical level, or perhaps talk about a simple case which is well understood that you are trying to generalise. The audience is very varied: I can guarantee you that most of the pure people have no idea what the Reynolds Number of a fluid is and virtually none of the applied people know what a ring is, so if you want people to understand you bare this in mind. A successful talk is one where the audience comes away knowing something they didn't before, having found what you had to say interesting. A talk where no one understands anything that is being said and which is being given purely for the purpose of stroking the speaker's intellectual ego is not a good talk (and I have sat through my fair share of them!)
It's basic stuff, but practice giving the talk a couple of times before you actually give it for real. When you come to give the talk it is far more enjoyable for the audience if you know what you are saying and you don't rely on your notes too much. I'm not saying memorise your talk, but just be familiar with it. We will have a lecture theatre with a projector, so you can give the presentation digitally if you want to, but I strongly recommend giving your talk on the blackboard. Using the blackboard is much more interactive and it controls the pace at which you deliver your talk, it is much harder to overload your audience with information if you have to write everything by hand. All of the really good maths talks I have seen have been delivered on a blackboard. By all means use the projector for videos or complicated diagrams, but when it comes to the actual meat of the talk, give the blackboard a go.