How can I update my rooted Moto G to 4.4.2 Every time I try the OTA download it gives me an error while installing, which I presume is because I've rooted the phone.

It's a European model if it matters.
You might have to (re-) flash the complete (stock) ROM. Rooting involves changing things on the /system partition, which changes its checksum. Some OTA updates use the checksum to see whether the update "fits", and thus will refuse to install if not. Looks like your Moto falls into this category.