Usually, the bank issues a full letter stating the start date, interest rate, active amount and parked amount with no specific timeline.
The small print will say the bank will do a review of the agreement on a three yearly(sometimes yearly) basis with the option to move, depending on circumstances, credit from the warehoused amount to the active amount and vice-versa.
The repayments on the active part might be a lot more than interest only/rescheduled payments they have up until now being paying.
A 6 month only split is very strange as why not just offer a moratorium/rescheduled arrangement instead. The suspicious part of me wonders if this 6 month split could be used to inform the central bank/banking enquiry that the bank in question is fully participating in addressing mortgages in arrears by offering "long term" solutions, with 6 months being the long-term.
May be an old fashioned misprint or, the review time is an ultra short 6 months, as the debtors might have informed the bank that their circumstances might change for the better really soon?