In the 70s I spent three years at Linton as a JP QFI, with no collisions during that time. There were many occasions when you knew that there were 50+ jets in the Vale of York, from Linton, Church Fenton, Leeming, Dishforth et al, all doing General Handling or Navexes, when often times you would never see one outside of the circuit!
The nearest we got was one of the mates (Jack D) pulling 6g to avoid two parachutists (who hadn't Notam'd their drop) plopping open in front of him.
The Grob accident was a most unfortunate anomaly.