Well after my shock of the adults killing my babies last Saturday, I have now had a pleasant surprise this morning. On Saturday I removed some 'newly laid' eggs from the tank and put them in a very small tank with a couple of inches of water. They just hatched out this morning! I counted about seven or eight, but there may well be more hiding under/around the plant.
I have no external aeration or filtration, but will be buying a pump/filter/airstone tomorrow. I am hoping that the plant will provide enough oxygen for the next 24 hours (please correct me if I am wrong).
Hopefully these babies have a bit more luck than their earlier brothers/sisters but at the same time I am concerned about how few there are. I guess that time will tell...