Someone asked me a useful question about 'semi-formal' letters. I've summarised the student's question here:
The task in my test was to write to a colleague. In my letter, I used some informal language e.g. I greeted the colleague in a friendly way and I used contractions (I'm, don't, isn't). However, I also included some formal language when asking the colleague to speak at a conference. Is this mix of informal and formal ok?
In this lesson I wrote that we can forget about "semi-formal" letters. However, in the situation described above, I think it's fine to include both formal and informal language. You can write to a colleague in an informal style, but you may use some formal language when discussing work-related issues. This is what we do in 'real life', so I think it's perfectly acceptable in an IELTS task.