Hello Gary, only way to get a refund is by canceling your AdWords account, see: https://support.google.com/adwords/answer/1703646?hl=en-GB
If you already canceled your account and you are waiting for your refund then be aware that Google does not provide refund in following scenarios:
- You have money left over in your account from a promotional code.
- Your account is still active.
- You have an outstanding balance that you still need to pay.
After 20 days of waiting you should just call on AdWords support and ask them about the refund delay.