We are having about the same lows in Pyrgos as well but day highs are around 20C and the bananas grow some,especially the ground planted ones. For this reason,if correctly done and ground planted in well draining,rich soil,slightly mounded,and care is taken to not disturb the rootball,ground planting it now would not put it in any disadvantage. The roots and corm would be better protected from cold in this manner,and its soil would drain from all sides to the surrounding soil instead from the few holes of the pot. In a healthy soil,worms and other organisms quickly tunnel through the disturbed soil and make the drainage even better!
I totally agree with Mitchel that a south facing wall is the best for overwintering potted!