The Observer article does not say that James and Cooper are different people. It doesn't really say anything about the alleged investment.
One possible way of squaring the two accounts is that the Walkers invested in James' property business. If they bought shares, they would have hoped for a return on their investment. However, assuming this is an investment in a limited company, not a loan, they would only get their original investment back if they were able to sell the shares back to the company or to someone else. Then, when Sally and Tim went on the run, they go to James and ask him to buy their shares back. He tells them that the business has failed (or is failing) and the shares are now worthless, so neither he nor the business will buy them back. However, he will lend them £100k secured on their house at 18% interest to help Sally avoid prison.
To me, that makes sense. Cooper is James. There is no loan to Cooper, but there is an investment which is lost when the business fails to perform. The loan to them is not related to their investment, it is due to Sally's criminality. It allows Sally to spin a story which has enough truth in it to mean she can probably justify it to herself as essentially true, but which hides her embezzlement.