I mean, if you have the funds/time/opportunity/effort/etc. to retake Eng Lang, it wouldn't hurt. That said, my understanding (if anyone sees this and can correct me here please do!) is that one 'bad' (a 6 is a strong pass, don't forget!) grade isn't the end of the world. Certainly when it came to my application (I did languages) I got a 5 in Maths for instance Cambridge has a very holistic application process (that is, they look at everything: your GCSE grades; your current predicted grades; your attitude and compatibility at interview; any extenuating circumstances at the time of interview/exams, past and present, to name a few) and if you're able to demonstrate a strong ability and enthusiasm for English Lit I would say you should be okay? The reason I bring up that it wouldn't be a bad idea if possible is that it shows of course your desire and enthusiasm to look at past mistakes, especially in a subject semi-related like Eng Lang! It's okay to improve, if anything they'll like that!