It is entirely up to you when you decide to talk to your employer and what you decide to say. You can get advice from your union rep or from a national charity about the best way of talking to your employer about your disability. For example, Action on Hearing Loss has a section on what to do if your employer responds badly.
You are not obliged to tell your employer about your disability but remember that if the employer does not know (and cannot reasonably be expected to realise) you are disabled, they are not obliged to make reasonable adjustments to accommodate your needs, and they cannot be criticised for treating you less favourably for a reason relating to your disability.
The Health and Safety Executive (HSE) says that the evidence is clear that early intervention with people off sick is key to achieving a successful return to work. This is especially the case with back injuries, other musculoskeletal disorders, and stress-related absences.