Originally Posted by Gin and Tonic Please

Just did this myself, and can confirm it works well. Never used developer mode before, but found it quite straightforward. For anyone else trying to figure this out:
1. Right click the offending tick box and select Inspect.
2. That brings up a large window of code (developer tools), which will have a line highlighted. Double check that contains the voucher number you’re trying to use
3. Delete the bit that says disabled=“disabled”, and as if by magic, you can now click it!

Thanks, aahjnnot!