Betty,
When I went through the assessment (about, blimey, 4 years ago now) we had compass tests, a group exercise, group discussion, interview & "sim check".
Compass test - if you did GCSE Maths and Physics and have mis spent your youth (like most pilot wannabes) playing Microsoft Flight Sim you won't have a problem with this.
The first group exercise involved being given a scenario of being marrooned on a desert island and deciding as a group how to prioritise what to do and prioritisation of equiptment salvage. Results then presented to assessors. Don't think they really care what you decide but how you interact with others.
Second group exercise was to build a bridge out of materials provided, part of which conducted in silence. Again, I suspect your ability to build the bridge to the specification required is irrelevent, how you work in the team is what's assessed.
Group discusson was around 2 topics, we were simply told a statement and then left to discuss it. Topic 1 was "With improvements in automation and GPWS, why does CFIT continue to dominate accident statistics". Topic 2 was surrounding a recent (in 2004) European Court ruling that affected Ryanair's subsidies to fly into a particular airport (I forget the details) and were asked to discuss how this court ruling would affect our future career prospects.
Interview - can't really remember too much about specific questions, but one thing that stands out was an unpleasant "good cop/bad cop" approach by the two interviewers. Not sure whether it was deliberate as I thought that had long been discreditted. Overall, a calm, understated confidence will get you through - no killer questions.
Sim check was very simple. The hardest it got was turns onto headings whilst descending to a certain altitude which would require rolling out and leveling off at the same time. We got a full pre sim brief that was aimed at assuming you had never seen a set of aircraft instruments before.
When I went through I asked about the pass/ fail rate and what the main reason for for failure was. They said the about 55 - 60% passed the compass tests and of those who got through the biggest reason for not getting all the way through was a little bit of imaturity/ lack of confidence which was especially evident in those fresh out of school.
Best of luck.