TC, service records were given a grading of confidential. As a Sgt your cousin may have had RAF Form 6442 raised as an annual report. I am assuming that was still the number in 1951. In addition, as aircrew, he would have had RAF Form 5000 series raised for each phase of training and annually on squadron. It is a series as different forms would be completed for different things. A post flight form during flying training and operational conversion unit training would have been raised after every flight. On the squadron a form would have been raised after every check flight by a squadron commander or flight commander.
Dave Thomson said you can apply for a copy of his records, I believe you actually need the permission of his next of kin so you may need to track down an uncle or aunt or one of his brothers, sisters, or spouses thereof, whoever is the next of kin.
Not mentioned so far is the Air Historical Branch at RAF Northolt who hold the squadron operational record books, Form 540, a narrative, and Form 541 that details some flights such as recognized exercise sorties as opposed to ordinary training.
Good luck.