IELTS is the test of your ability to speak, write etc. English, right? Although Sussex are sometimes willing to let people in with less than their offer I'm not sure they'll think your English is good enough to come. The only way you'll find out is calling the uni though.
They can offer you a pre-sessional course if your results are not lesser than 6.5 not more than 5.5 in any of the components. However, this pre-sessional courses never guarantee your place in the course so I suggest you to get on with your IELTS score beforehand. Good luck!