First question, are you expected to pay like any delegate would?
Usually, yes. Generally (in my field in the Humanities anyway) only keynote speakers have their expenses paid.
Secondly, I am doing a 20 minute paper presentation (although they state this is not an actual paper and they just want my ppt slides), and it is on my MA research. I am due to complete and submit my dissertation in August so is it ok to discuss my initial findings part way through?
If you're confident about your part-way conclusions & argument.
The main main main thing is: DON'T GO OVER TIME. Even if you rehears & you're on 20 minutes. cut it back. It always takes longer giving a paper than in rehearsal. Aim to speak for 15-18 minutes, and you'll be comfortable in a 20 minute slot.
Going over time is the biggest mistake I see in people unused to giving academic conference papers. It's just plain rude, but even more, you're robbing yourself of the feedback from questions & discussions.
It's this feedback that is the thing one goes to conferences for - the debate, the discussion, other people's contribution and collaboration with you. That is priceless (as they say!)
I think the mistake comes from inexperienced research presenters wanting to tell me all about their process & minute detail of argument - I'm actually not that interested in those details. What I want are the ideas, the hypothesis, the conclusions, and the further significance of those conclusions.
In other words, cut to the chase - this is what's interesting to other people about your research.
So other tips: think of your audience. What's the hook of your research into the field? Cut to the chase about this.
Breathe. Take your time, although no long pauses. Look at us. Speak clearly and with a hearable rhythm and cadence. It helps to sit up straight, or stand up straight (depending on whether you're standing or sitting). Shoulders back, chest open - this all helps you to breathe, and this speak clearly & in a pleasant hearable rhythm. I don't mean volume - although that as well - but a rhythm of words and delivery which makes your sense intelligible. I use punctuation to help me with this. My conference paper texts have lots of dashes in them, to help me phrase & breathe.