Poor babies! They've definitely had a rough start. I'm fairly certain now the issue has been too much moisture in the soil. I believe there are a few factors that have had an impact:
1) Obviously, giving them too much water. In the first week I gave 90ml into their 1 litre pots over two waterings. I thought this was okay because the top soil was completely dry between waterings. Someone suggested that seedlings don't need additional water for the first 10 days so I'll try that next time
2) Feedback I've had is that my soil mix is quite heavy @ 63% soil, 25% Coco, 6 % perlite + 6% vermiculite. In future I'll increase the perlite portion to about 40%
3) Also, I just noticed that the plastic pots I'm using have very small drainage holes. I think this has caused moisture to accumulate at the bottom of the pot. Yesterday I pushed my finger all the way to the bottom of the pot and it is much wetter at the bottom than at the top.
So yeah, tough start. But they're still growing, albeit very slowly. I trust they'll survive.