No. You do not have to send a letter of demand, it is trite law that notice must be given. Many creditors send numerous letters of demands to their debtors but this has proved to be a waste of time and expense.
If payment arrangements are broken you have every legal right to start action. You need no notice except if this was a credit agreement and the parties agreed to notices therein but this is not compulsory by law.
Creditors find themselves in a predicament by not hand over as soon as promises by debtors are not forthcoming. The longer the debt is outstanding the longer it can take for payment. If the debtor had the money in the first instance you would have been paid. What you now need is a debt collection company to prioritise the debt so your debt can be first in line for payment if cash becomes available.
Holding on to the debt only makes this process longer and extends getting paid for longer periods.
There is nothing in law that enforces any responsibility on the credit provider to notify you that you are being handed over. It is trite law that notice should be given. If you failed the first payment agreement date the debt becomes legally due and payable and you have defaulted in your payment arrangement.