Anyone previously with a CIS registration should remain in the Inland Revenue's database until you tell them otherwise. This makes it easier for contractors to verify your employment status.
For the newly self-employed, there is no longer a CIS system.
You still have to declare yourself to the taxman as being self-employed (small business, sole trader, partnership etc.)
It is now up to the contractor to inform the taxman that he has employed your services. He must also provide accurate details concerning the kind of service you are providing.
It is all to allow the taxman enough information to decide if you are truly self-employed, or if you are, in fact an employee of the contractor (in which case you will be liable for more tax)
The simplest way to prevent this, is to ensure you have at least 2 contracts with different contractors in any 12 month period. Ie: You are being paid from 2 different sources.
If you are not yet registered as self-employed, you should do it straight away or your contractor will not be allowed to pay you until your employment status is verified. As mentioned above, larger developers will definately insist on sticking to the new rules.
Simply pop into your local tax office. It only takes about 20 minutes.