In September I broke my foot at work. I was originally signed off for 8 weeks but as there was no one to cover my job I started back with no support from my manager
Was your accident formally recorded as an incident in their accident book or whatever records they keep. Irrespective of whether your accident was as a consequence of some negligence on their part, they still need to keep records.
As regards returning to work before you were fully recovered, that was a decision you made, and whilst it is commendable that you tried to get back to your job, it wasn't necessarily an action that you would ever be thanked or recompensed for.
I would summarise the circumstances of your health in writing, stating that you have struggled with your MH for several years including the OD 3 months ago, and you are committed to return to work, possibly on a phased return for a month (or whatever timeframe you and your GP decide is appropriate), and you would request their support to help you to return to full duties.
I wouldn't mention the bullying manager, as that would be a hiding to nothing Im afraid. You'd always come out the bad person. Keep the letter/email in the positive, and state clearly what they can do to support you. You could (as I advised to another poster on here) set up a spreadsheet which logs your daily tasks broken down by 15, 30 or 60 min timeframes to show what you're working on and any tasks that cannot be completed due to lack of time. Suggest a meeting to discuss, as it could prove the point that another person needs to be recruited, even temp/agency to cover the backlog.
I can not detect anything they have done that is necessarily discriminatory, particularly as they did initially support your disclosure about BP, but it does seem like your workload has become excessive, so you need to prove that with facts and data.