Disabled Students Allowance evidence

Hi! I recently became aware that I'm eligible to apply for DSA due to mental health and ADHD.

How would I go about getting evidence? I have a previous diagnosis of the mental illnesses I have applied in relation to but this was from around 2020, would I need to be re-diagnosed? I have never received a formal ADHD diagnosis, but this is something I've had the exams team at my college agree with and apply for exam arrangements for my A Levels due to. I assume I'd need a formal diagnosis to get help in relation to that, but what's the easiest way of going about this?