(2-part post):
There are certain circumstances when bailiffs can force entry into a house. (They can do so when attempting to recover unpaid court fines and, subject to obtaining a court warrant first, when trying to claim unpaid taxes and duties which were payable to HMRC). However the power to force entry does not apply to recovering debts on behalf of the CSA.
If bailiffs are likely to call, it's important to ensure that there's no way they can get into the house. If a debtor agrees to let them come in "to talk things over" they're then entitled to seize goods from the debtor. Similarly, if the debtor leaves a door or window open the bailiffs have the right to enter the property and seize goods.
There's some relevant information here. (I've linked to the most relevant page but, to 'get the full picture', it would be best to check out all of the links on the left):
http://bailiffadviceonline.co.uk/csa_csa.htm
(NB: That's a commercial site, which is hoping that users will pay to download their forms and letters. I'm not suggesting that your brother should use those services. I've only linked to it because it's an excellent free source of information).