I'm using the now app for which I pay monthly for the entertainment pass on our smart TV. I can access other programmes which are on the Now app but not Gangs of London.
I've uninstalld the app and reinstalled it, I've cleared out the cache etc and checked that my subscription is still up to date. Not sure what else I can fo,
You could try unplugging both your TV and router for a few minutes.
Otherwise I would suggest to report the issue with staff, at least then you’ll covered the main troubleshooting that they would ask you to do. All that is left then is to do a factory reset your TV which is on the extreme side, it might fix it though, but obviously would be a pain to set it up again.