There's serious doubt about the date of this one. There were many somewhat similar incidents in this area of the UK around this time. May 20 1957 was arrived at by elimination. When Torres compared log books with the pilot he recalled having been his flight leader that night, this was the only date that matched. However the details of the interception remembered by the other pilot, Dave Roberson - see http://ufoupdateslist.com/1998/jun/m07-034.shtml - are significantly different, and although these two usually flew together they did not do so all the time. Subsequently Torres concluded they were talking about different incidents:
"I have talked to Dave many times since the revelation was made and I am sure we are not talking about the same mission. . . . who ever was on duty that night was in my flight. I assumed it was David. . . . David does not remember these facts, he remembers being split and he would go lower. This was not my intercept. We had a lot of weird activities at this time and it might have happened on another occasion but not this one. . . . Try as I may I can not remember for sure who was flying the other aircraft or even if he is still alive today."
(email from Milton Torres to Dr David Clarke 15.03.2002)
Therefore the date of May 20 is unreliable.