This is another step in the right direction taken by the HMRC to help taxpayers. According to the HMRC web site, completing the ESI tool will provide an indication of your employment status. Click here to try out the ESI tool.
The answer can be relied upon as evidence to support your position, provided your answers to the ESI questions accurately reflect the terms and conditions (not just the written contract) under which you provide your service and the ESI has been completed by your engager. If you just complete the ESI tool the result is only indicative.