They couldn't find the IOU for the loan, but they found a blank financial agreement, so they got the parents laptop and found that she had loaned the money to the publican's wife to set up her catering business. When the husband barred them they asked for the money back and she didn't have it.
She murdered the couple, the man who witnessed the agreement (only found that out near the end) and then she murdered Freddie because she know about the loan too.
Then she got her husband, herself, barnaby and Jones and threatened to blow them all up, but the gas in the gas bottle ran out as the husband hadn't filled it! So she was carted away!