Hello, I am a solicitor with 20 years experience. I will try to answer this for you.
This is a hard type of case to win. You are likely tp need ot show that yo have an illness that makes it hard for you to breath into the intoximeter or that it may have been defective. This second route requires triggering extra disclosure from the CPS than they would usually provide (g of maintenance records etc.) and of understanding these when you get them.
You can represent yourself but I would not recommend it. Find a solicitor you do have conficdence in, you do not have to stick to one solicitor if you are unimpressed.