I can help, I was at the meeting. It was some kind of trial involving six teams of sidecars which made the Plough Lane track impossibly small. I think it was a normal speedway meeting with a number of special sidecar races... might be wrong. We used to go to midweek mtgs from Woking to see my hero Olle Nygren. Can't think we would have gone up for such an oddity which I never saw repeated.
It was clearly too crowded obviously extremely hazardous. In my view it should have been halted very early on. There had been a number of crashes, near misses - on the fateful race, whilst I cannot remember all the details, I was sat with my parents in the restaurant opposite the start gate. What happened was that coming out of the bend approaching the start gate (can't tell you which lap) the bike and side car lost control, Chris was in the car, the bike hurtled towards the start post and Chris hit his head on the gate pole or at least an upright pole near the start. He lay there surrounded by medics, was the carried away. The race was run again! They announced his death at the end of the meeting.I was only a 13 yr old child but so moved I wrote a poem called 'Ode to Chris Jackson' - won't embarrass anyone by repeating it but I still have it all these years later. It made a huge impression on me.
Hope this helps