Air North Website at
http://www.airnorth.demon.co.uk/section3z.htm
Preview of an article from January 2003 - Full article available on request.
"Escape From Eight Miles Up:"
The amazing story of a mid air collision between two RAF Javelin all- weather fighters over Hartlepool in May 1960.
8 miles up suggests 40,000 feet so most likely visual (VMC) ... but whether or not they were operating under IFR (eg night?) is another matter.