Just had my monthly statement in. There was seven quid interest on it. Since I pay the full amount each month by direct debit (on the date they request it), it would be quite difficult to rack up interest (I don't use the card for cash withdrawals). I rang their Customer Service department to query the interest: "Oh, we've had a few problems: I'll credit the amount to your next month's statement".
Aside from the issue of giving them an interest-free loan of seven quid this month, the moral of the story: if you have interest on your Morgan Stanley Mastercard account this month (especially if you don't expect it): check it and query it.