Agree that if you want proper advice, go to your nearest Citizens Advice.
But the debt collection agency asking for the income and expenditure information have no rights to receive this. Only a court can request this. So make an offer of an affordable amount, continue to pay it and the debt company will just have to accept it.
While you are in contact with them, ask them for an up to date statement showing how the current debt amount is made up. If they are adding any interest or charges, it may well be incorrect for them to be doing this.
Who was the original debt owed to ? What type of debt e.g loan, credit card ? Do you have a copy of the original Consumer Credit Agreement, plus terms and conditions of the account, a copy of the default notice and any termination notice. I think I would be having a look at these. Request a copy of these from the original creditor, if you don't have these, by sending them a Data Protection Subject Access Request (SAR)
If the original creditor applied any charges or PPI to the account, you might be able to claim these back. It would help, if you had all the statements of account from the original creditor, so asking for these by making the SAR would make sense. Who knows you might find charges/PPI which can be claimed back, which might reduce the debt owing.