Whirly,
No one is jumping to conclusions, they are merely speculating on the cause which as I have pointed out is healthy. If speculation causes us to think about our flight planning then it might just save a life.
I teach on one of these aircraft and can assure you it is very performance limited. Looking at the photographs of the deceased they look like the average overweight 50 plus brits. The teenager maybe have been a rake but even without him 3 adults take the aircraft out of the W&B with all but the most minimal of fuel. They were heading to France for what I speculate to be a trip so will have had baggage however minimal. Factor in density altitude, rough grass, sloping surface and weight and the POH in front of me has them way outside the envelope. So I would therefore SPECULATE that they were overweight, had not carried out the correct factoring calcs for the surface or the DA, pulled the aircraft into the air before it was ready to fly and the aircraft wallowed around on the edge of the stall falling to climb. Possibly in an attempt to get the aircraft to climb out ground effect the nose was raised just enough to turn the wallowing into a stall down it comes.
I would further speculate that as they took off from a tarmac runway of a similar length that they felt emboldened enough to attempt the same thing on a rough grass runway.
Sandown is not the most friendly of surfaces for performance limited aircraft and the optical illusion created by the runway being in a bowl would not help the situation.
This is just my speculation but I am prepared to take bets on the AIB report outcome.
So what is the outcome of my speculation? Always do a W&B, always check the DA even it seems irrelevant and always carry out the surface factoring as recommended by the CAA.