So your chronic condition hasn't affected you at work to date but is starting to now, is that right?
Do your employer know about it?
In terms of occupational health, they are experts in medical conditions as they relate to the workplace, so should be able to advise on how a condition might affect a person's job, and what if any adjustments could be made to that job to enable that person to work effectively.
They don't just work in house, many of my clients are very small but I have used external OH providers to advise them on health issues when they have arisen with an employee. Obviously there is a cost involved however so with a small business it's unlikely they'd just refer you based on your request if as far as they are concerned everything is currently fine.
I think the first port of call should be your doctor, if you haven't seen him/her already, to discuss your condition and how it's affecting your work. GPs these days are expected to write fit notes which would involve specifying what duties an employee is fit for, rather than just signing them as being off sick or not.
See your GP, see what he/she says about the issue, and take it from there. But ultimately if your condition is starting to affect you at work, you are going to have to talk to your boss about it at some point.