Just thought I'd send an update. More bad results from 'in vitro plants', but some new lessons learned.
Before this attempt, I wrapped the tank in clingfilm leaving a small gap and used a heater in a bowl of water to create humidity, but things got very moldy and the plants died within a week or 2.
This time I did it without the heater and misted the plants twice a day, however I think the water level was too high - only about 1 cm from the surface. I think this, combined with not enough air exchange and too much humidity was the problem.
I went from this:
To this: (after taking out all of the dead plants)
So I'm back to square one again, but at least the other plants have survived and I only need to replace the HC.
I think the trick is to create humidity without the temperature. I have some cuttings in a sealed container sitting next to my very cold kitchen window, which get hardly any sun, and they are doing just fine.
I've ordered some more which will hopefully turn up today. This time I'm going to drain the tank completely and leave some very large gaps for air exchange. Will let you know how it goes.
If anyone has any other tips that might help, it would be greatly appreciated!