I was in social work for a number of years, in the end, it burnt me out.
Catherine, I agree with Smokey, they can't sack you due to ill health and I would also agree that it doesn't matter what OH are saying...anyway, who are they to say you won't be fit for duties for another 18mths? Are any of them Psychiatrists? Do they know something we don't? At the end of the day, that decision is down to your GP and you. I suppose, being a local authority, they might consider suspending you if they were adamant you were not yet fit enough.
The thing is, you said yourself, things were going okay under the phased return, but then you did start to feel low again. People don't appreciate just how difficult the job is. The last thing you want is to reach burn out. Is it possible to go back part time for a while? Are you meeting anyone for support while you're off? Have the OH been providing you with therapy of some sort?
I hope you manage to sort something out. Welcome to the Forum!