Group Income Protection & ESA

I have been on my employer's group income protection (gip) for over 18 years and have always claimed incapacity benefit alongside this. I was automatically migrated to ESA some years ago. I continue to be employed and the gip is an employee benefit paid through payroll, and reclaimed from their insurer by my employer. They also deduct the rate of ESA. I have received a letter from DWP requiring me to undergo a telephone compliance interview. Having spoken to a member of DWP they are stating that because I haven't paid over 50% for this income protection my salary will be taken into account and I may be required to repay 18 years worth of benefit! My understanding is that as I am employed status and am paid by my company this isn't the case. They have the insurance policy with L&G not me. I am not retired or terminated. I pay tax and NI and my employee also deducts the rate of ESA (around £5k per annum). I pay pension contributions and receive employee benefits such as private healthcare. How can they be suggesting this? Surely my gip should be counted as contractual sick pay which is neither earned income nor a pension. Any advice would be gratefully received.