Well I have finally had this reversed and the expnation for it is unbelievable!
A few years ago I applied for Carer's Allowance because I look after my mum (she has vascular dementia). The result was that I had an 'underlying entitlement' to Carer's Allowance, but that my ESA cancelled it out. So I just put that aside and carried on.
What happened earlier this year is that someone saw that I had applied for it, and had been entitled to it, so they put it down as income received, even though I've never had it. The amount that has been being deducted from my ESA is this.
*enter expletives and eyeroll emojis here*
Good grief.
Thank you all for your help. I wanted to update you all on the nonsense!!