I am currently a recipient of an ESRC 1+3 quota award for fees and maintenance stipend however due to various circumstances I may not be able to continue my studies. Does anyone know if I fail to complete, will I have to pay back the funding that I have received thus-far?

Probably not. I left a PhD and was only asked to repay a few days worth of stipend, as I was paid in advance and didn't quite finish one of the payment periods.