I went to go and put some tin foil on top of the perspex to reflect some of the heat back towards the eggs, and the thermostat kicked back in. I don't know what happened or how it managed to work again, but the best thing is that it is up and running!
I hope that the eggs will hatch now and weren't left too cold, it'd be such a shame and a waste if they didn't hatch :(
The adult quail are fine, they seem huge compared to when we got them and I can't wait untill they are laying.
Oh and I now have a hygrometer, which has been in the incubator for a few days and seems accurate :D