Morning @KernowChris, I got the below info on the BT.com faq's, hope it helps.
Existing NOW TV passes
Some customers may have already paid for a NOW TV Pass that has time remaining on it. If the pass you have paid for is also included in your new BT TV package, NOW TV will give you a refund to the next renewal date.
The payment will be refunded automatically using the same method you originally used to pay for the pass, so you don’t need to call NOW TV to ask for your money back. Refunds can take up to 28-days. We will bill you from the date your BT TV promotion starts.
If you want to keep a pre-paid NOW TV Pass, you can bring it with you when you join BT TV and it will be added automatically to your package. NOW TV will give you a refund and you will billed from the date your BT TV promotion starts.
Alternatively, if you don’t want to bring your NOW TV Pass to BT you won’t be able to access any content when the pass expires.