Hi Graham, sorry to hear of the troubles. We have thousands of successful workouts logged on iOS every day, however we do know of a current bug that especially affects certain iOS devices like an iPad Air and iPhone 6 which may cause the issue you’re experiencing. We’ve got a potential fix ready to get into the app store, and we hope to see it be available within the next 4-5 days.
Aside from the 7 day trial, we do allow a free 25km/month free on Zwift with no subscription necessary, so I hope you’re able to give it another try once this (hopeful) fix goes live.
Sorry for the inconvenience.