How to evidence a request for flexible work under reasonable adjustments

Shell2014

Hi everyone I've worked full time at the same company for 20 years. I've been on the sick for 5 months due to various mental health issues and possible autism. I'm also a single parent carer to my autistic child. I'm returning to work next week as a phased return. OT has recommended this and also flexible working under reasonable adjustments as my mental health falls under the disability section of the equalityact. My manager immediately said no! Which wasn't a surprise as he refused me 2 years ago when I applied as a parent carer. I've spoken to HR who have told me they have to consider it properly and to apply but cant promise the result as the decision will be made by my manager and ultimately I know he will still refuse. I just don't know how to evidence my request more than I already have. The company's own Occupational therapist recommended flexible working, can my manager legally refuse this? I've got my union involved and they don't know any more than me. I just don't know what to do any more and don't want to lose my job over this but I know I won't be able to cope. Any advice would be much appreciated thanks x