I fixed the machine (black) hooked up to the monitor labelled OAK. It now boots and loads fine.
The only remaining problem is it has no internet. This is due to what looks like a broken ethernet cable, but as all the other action room PCs are in use, I can't test this right now.
Dawnsio 'mlaen i'r gwres prynhawn a rwy'n addo erbyn yfori byddai wedi mynd, hwyl fawr i pawb, hwyl fawr i pawb...