Good Evening Paul;
According to the Google Specs page http://www.motorola.com/us/shop-all-mobile-phones-1/Moto-G/moto-g-pdp.html#moto-g-pdp-specifications the phone takes a Micro SIM. Hope this helps.
A micro sim is what you'd need. If you haven't done so already, why not order a free giffgaff sim by clicking a banner so you'd get free credit but also receive the extra benefits giffgaff offers!
The micro sim is what you would see if you pop it out from the standard sized sim when you order one (if you've already got one, then perfect, you are good to go!)