Andy
Welcome to the forum, you might want to have a look at the following FAQ which explains where everything is
Welcome to Benefits and Work
Were you only receiving Income Related ESA?
If you are then the earnings limit is a fixed line, if you exceed the limit then you are considered to be in remunerative work and no longer have Limited Capability for Work.
Is there any debate to be had about what you have earned? Is there any way that you can argue that the amount were incorrect? Was it a one-off or did you exceed the limit for a period of time?
Gordon