I linked to the XDA thread by t-bon3 - which is a great guide.
Here is how it went for me.
First issue that the device is locked to tesco sim cards.
Note: you are not unlocking the bootloader or adding root to the system.
The phone will be exactly as if you bought it from retail, just you got it subsided by Tesco.
I choose these guys to get an unlock code - they seemed professional - I avoid ebay & paypal.
Phone Model : *Moto G*
Country & Network : *United Kingdom - Tesco Mobile*
I then got 3 Emails.
1: Confirm Order
2: Confirm Payment
3: Unlock code (came 11 minutes after I paid)
There were 2 unlock codes - I put a non-tesco SIM in and was greeted with the prompt - the first code worked.
Then after that I followed this guide to Install the UK Retail firmware
Files From here
I used this one
Simple task, enable USB debugging, reboot into fastboot, run script, reboot.
Now just to wait for the Over The Air 4.4.2 update that is due any day for the UK phones.