You need a coin-cell battery (CR2032): CMOS Battery on the Intel® NUC
You need battery with pig-tile and connector. This is example of such battery: 1 CMOS Battery CR2032 For HP Pavilion DV6000 DV6100 DV6200 DV6300 DV6500 DV9000 | eBay . There is a lot other suppliers online.
Hello Leon Thanks mate! I will have to order one from the UK.