Please may I ask for some advice on proving a debt?

[mumisskint]

My X Lent me some money secured against my home. the contract between us stated more may be lent over time which it was. he is now taking me to court to get that money back even though if he wins the kids will lose their home. thing is he has doubled the amount he says I owe. he won't sit down with me and go through it, he's not produced bank statements to the court showing transactions just a spreadsheet that he has made. there are double entries etc. can I ask the court to make him prove each transaction? I've been up reading the CPR rules all night - when is a section 31 appropriate? does the burden of proof shift to me if I'm challenging the amount he says I owe even though his evidence isn't clear and concise? does the court have a duty to establish the facts before making a decision? if there is a dispute between parties what happens? I'm so scared! any advice / ideas?