As a broker, we've had experience in the past with Irish Life and Aviva for paying Income Protection claims. Irish Life were difficult to deal with and tried (unsuccessfully) to make out that the claimants weren't genuinely sick. Aviva were far more helpful and paid the claims without quibble. But that's only our experience and might just have been because of the specific details of the claims involved.
Some claims are black and white. If you're a surgeon and break your arm badly, there's no question about your inability to do your job. However, if you work in an office job and are suffering with depression or other mental illness, or perhaps chronic back pain, it's also keeping you from being able to do your job, but from an insurer's perspective, it requires more evidence that you're genuinely unable to work and not faking it.
I'd second the advice of
@Steven Barrett above. You should ask your own broker to help you to submit your claim.