I don't know what the competition is like really, as I don't know who else/how many other people applied - all I know is that I applied and luckily got one! I reckon if you think you will get AAA then you have a reasonable chance of getting at least the £2000 award. I don't know about sciences, but for engineering most of the offers were quite a bit lower than this, so I worked on the basis that hopefully there wouldn't be too many people with AAA applying/making Exeter their first choice.
I can't really remember exactly what I wrote about, it was quite a while ago. I think I wrote briefly about my GCSE/AS results, then went on to show how I was interested in my subject (Engineering). For example I said a bit about work experience I had done (paid summer job at an architects, and one week with a civil and structural engineering company), and also about a technology scholarship I'd received for sixth form, and about the Schools' Aerospace Challenge I'd entered with a team and reached the National finals.
I think I also wrote a little bit about my non-academic interests, such as playing in a brass band, music exams I'd taken, and various sporting acheivements. Not sure if they needed/wanted to know about that, but I thought it was best to give the impression of being a well rounded individual, not just someone who works all the time!
Hope your school write you an OK reference! But then if they make mistakes with typos and stuff then the uni must realise that that's not your fault at all...
In terms of what I think of Exeter, I really like it here. If you haven't looked round yet then I really recommend coming on an open day. I know lots of people who weren't even seriously considering it until they saw it, but then made it their firm choice. I was a bit apprehensive because the grades they wanted were much lower than I had, so I thought it wouldn't be challenging enough, but as I don't like big cities and I wanted to stay fairly local I didn't have many choices. As it is I'm definetely glad I came here. The work admittedly doesn't push me as much as it might have done if I'd gone elsewhere, but it is still interesting and worthwhile, and it means that I get enough free time to continue with lots of music/bands etc.