A default notice is the first stage in the procedure towards action in the county court. It doesn't automatically mean that court action will follow. However, sending you a default notice opens the path to court action if the lender chooses to take that path:
http://www.payplan.com/debt-library/debt-manag ement-default-notices.php
Normally, a lender won't rush to take you to court after issuing a default notice. They're more likely to try to get you to enter into a payment arrangement, either directly with themselves or with a debt recovery agency appointed by them.
If the matter does come to court, a debtor is required to complete forms, detailing his/her income and expenditure. The court will then issue a judgement (a 'CCJ') stating the amount which the debtor must pay back each week (or each month). That amount has to be within the debtor's means and then, as long as the debtor complies with the arrangement, the lender can't send the bailiffs in or seek to make the debtor bankrupt.
If you've received a default notice, the Consumer Credit Counselling Service recommends that you don't enter into any agreed payment plan, until you've spoken to one of their counsellors. (I can recommend the CCCS. They've got really knowledgeable, helpful, friendly staff who can put your mind at ease about debt problems):
http://www.cccs.co.uk/
Chris