This morning before heading into work I did a quick count of the fish and noticed only nine. Fearing one had jumped out I was looking around the tank for it when another tetra jumped out right in front of me straight into my hand. It was jumping all over the place but I managed to pop it back into the tank without delay.
Water parameters seem alright, low nitrates, zero nitrites and ammonia, low CO2 levels at the time it jumped. Water level was about 1-2cm below the rim of the tank. They are very skittish fish and dart around really fast come feeding time. Doesn't help that they're situated near a doorway as anyone walking by sends them darting for cover.
As the plants grow it should afford them more privacy but until then it looks like I'll need to get the cover glass otherwise I'm losing £10 everytime one of the buggers jump out!