As a fellow UNISON member and one time shop steward I know that the Union takes depression very seriously, as should any decent employer.
I also think that honesty is by far the best way to deal with these kind of problems, bullshitting won't wash.
I work for a major utility ( water ) company and they just crap themselves when stress or depression is mentioned incase the employee say's it's job related.
My manager knows about my history and about my depressive times, we work together to make everything go as smoothly as possible for both of us. But I'm lucky to have a good manager at the moment, the last one was an arse who just got on my case. But in the end I won through because I never once bullshitted him or made excuses.
Go confidently in the direction of your dreams! Live the life you've imagined. As you simplify your life, the laws of the universe will be simpler.
Henry David Thoreau