I must admit I can't remember the £600t you mention. Could it be some debt converted to equity? Or simply cash for equity to comply with SCMP (FFP)?
10% interest rate sounds pretty high, but ask yourself what rate you would demand for loaning money to the club? In any case, the creditor will only get the money back (loan plus interest) if the club ever manage to produce enough profit. Not really likely to happen any day soon, so they could just as well charge 20% or 50% interest rate - it's of no real consequence as long as interests are accrued to the debt.