As I understand it, ESA is paid to claimants with disabilities who have demonstrated that they are unable to work. There are various categories within the assessment, but they boil down to the same outcome - an ESA payment that is a top-up to the basic JSA allowance.
So what I don't understand is how you are able to claim this when still employed, because I thought that it was for folks who are unemployed.
I imagine this is down to your employer's sick pay scheme having run out, and you following back onto Statutory Sick Pay - and this amount is less than the JSA+ESA total amount you would have been on, had you been unemployed.
Suggest the local Citizen's Advice Bureau or one of the charities for disabilities advice may be able to help you further. This one, for example.
http://www.disabilityalliance.org/index.htm