Can you ask for a reconsideration for ADHD assessment?

I'm an Autistic (official diagnosis of ASD) woman in my thirties, after an 18+ month wait I've recently had an assessment for ADHD - I was denied the diagnosis. 

I have severe executive dysfunction problems, severe enough that I can't hold down a job or go to higher education, I can go days without sleep, etc. also problems with focus so I struggle to read more than a few paragraphs at a time and can't even sit through a few minutes of a video, among other traits - autism explains the executive dysfunction, not focus issues. 

I was seeking diagnosis to help with benefits assessments (I claim ESA-WRAG) also for support, as obviously options like ADHD meds can't be accessed without diagnosis. 

I'm wondering if I can appeal the decision or ask for another opinion, given the waiting list and potential cost to the NHS I'm not sure if they'd even allow it.