Hello and thanks for using Just Answer.
My name is ***** ***** am happy to assist you with your enquiry.
I am afraid this is a fairly standard request, and they are entitled to request evidence of your Mother's capital as far back as they deem necessary. As you ar eprobably aware, they just want to check the level of capital your Mother was holding and if it turns out your Mother did not disclose the current amount, any overpayment can be re-claimed from her Estate even if the non disclosure was purely innocent.
If there has been an overpayment, it will be classed as a debt of the Estate which will need to be repaid.
Sorry this isn't perhaps the answer you were looking for, but it sets out the legal position.