There is no need to worry as this is not a legal notice; it is simply a notice from your creditor to remind you that you are not up-to-date with your payments.
That said; a default notice should not be ignored. Some creditors will use a default notice to illustrate the first step in legal proceeding. According to the Consumer Credit Act 1974, a default notice must be issued before any legal proceedings begin.
If only a couple of payments have been missed, fill in a financial statement to find out how much money you have left over to contribute toward your debts. Contact your creditors with this form and make them an offer. If your offer is refused, continue to make your payments because it shows that you are willing to manage your debts. Then contact your local Citizens Advice Bureau for further advice.
If you can not afford to start paying off your arrears or have other mounting debts, you still need to act. Talk to your creditor as keeping them in the dark will only worsen the situation. In the mean time there are many organisations designed to provide free debt advice, so get in contact with them. They will help you manage your debts and give you immediate advice on how to help stop any further legal action from your creditors.