The only possible problem with the Robinson loan is an alleged potential conflict of interest. The DTI Permanent Secretary sought to insulate his Secretary of State from the Robinson inquiry on the grounds that he was a fellow minister. So there is already deemed to be a conflict of interest where fellow ministers are concerned.
There is meant to be. These people are in the same party. They are close friends (or at least meant to be). They are colleagues on the same side in the total war that constitutes adversarial political life. There is open loyalty between fellow ministers which makes impartiality not only improbable but impossible. It is not expected to be otherwise. They dine in each other's homes, go on holiday together. We should be amazed if they did not lend each other money, from time to time, as well.
Had the minister borrowed money from anyone other than a fellow minister (or a member of his own family) then the need to declare it would be understandable. As it is, we would not be in the least enlightened by disclosure of loans between senior politicians of the same party. To think otherwise would be naive.