firstly unis typically don't like it if you haven't done your a levels in one sitting, therefore if you were going to retake one, id say retake them all. if you haven't been formally registered in any of the exams you should be okay. are you in yr12? or y13? I'm going to assume the former. what do u want to go into and what unis are u looking at - if you want to go to university, u must look at their specific requirements you must also look at the likelihood of you being able to improve your grades. id say doing an extra year is not really advisable, especially if you don't have any mitigating circumstance.