ESA whilst receiving employee re health insurance

I receive top rate ESA due to a number of chronic illnesses I recently developed. I was paying into a group health insurance scheme through my employer whilst working for them. I stopped work when I became unwell. I'm still employed by them but don't work. I now receive monthly health insurance payments via a private insurance company linked to the employee group scheme. This is about £600 a month after tax and N.I on top of £125 a week ESA. I started getting the health insurance after the ESA was accepted.
I think now that I should not be getting that top rate whilst receiving private health insurance. I didn't declare this and have been receiving it for about 10 months.
Can you please advise as to whether I would or wouldn't be entitled to this rate. Could you also go about advising as to how I might raise this with DWP without getting myself into serious trouble.
To be honest, I've just felt so unwell with health issues and stress I just put this to the back of my mind.
If you are able to explain in understandable language that would be really appreciated.
Thank you in advance

Hi Jo and Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is

Welcome to Benefits and Work

Health insurance payments can be a difficult situation. In part it seems to depend on whether this is something you took pout entirely on your won or through your employer.

Either way, you need to contact DWP and let them know as it saves a lot of trouble further down the line. The first £85 per week (gross) will be discounted and then your ESA could be reduced by 50p for every extra £1 you get from the insurance. So, even if there is going to be a reduction, it is unlikely to be much.