If anyone else is using the new Stripe payment gateway, then the following might be useful:-
I tested (quite extensively!) the new Stripe gateway in test mode and then flipped it into Live mode, applying the new live API keys from Stripe.
One off, non-recurring donation payments worked fine.
However, if I tried to make a recurring subscription I got the following error message:
WARNING - No such plan: 8-1000; a similar object exists in test mode, but a live mode key was used to make this request.
It seems that Stripe has an issue with plans with the same name being created in test mode and then a subsequent request to create the plan in Live mode.
The answer is to log into your Stripe dashboard, select the Account Settings option, click on the Data tab and then choose Test Data->Delete all Test Data. This will remove all test transactions, subscriptions and plans.
This fixed this issue for me.