Right having weighed the empty cars, I can say that the spectrum ones are more than double the weight of the basic one (which weigh 155g). My load weighs 66g - 40g more than Big Jacks!
However, this still brings the all-up weight of load and basic model to around 221g - less than 2/3rds the weight of the unladen Spectrum car.
I did a test run (brilliant sunshine

) assisted by Thomas, who has now taken to ambushing trains!
There were no problems through R1 points apart from a bunch of water-laden daffs derailing the loco (I missed these when moving the bulbs from under the trestles!).
Looking at what I did to make the load, I'm not happy with the way the edges grumbled when I removed the clingfilm. This is down to the size of the stone (larger pieces tended to run down to the edge when sprinkled on the PVA) and maybe the glue was not completely set.
On balance, I'd say the Vermiculite was the best way to go!
Since I have the Spectrum cars out, I'm supergluing the check chains in position of the chassis, otherwise it would be rather tedious setting them up!