UPDATE: after trying everything I could think of (and what was suggested in this forum) I finally gave up and contacted the dealership. They took my details, phoned me about 5 days later and told me it was all sorted. I didn't even have to re-install the app, I just logged on as I always had and voila it worked!
To anyone having issues, it may be worth doing the same. It's a pain having to go to the dealer but they seemed to know that this was a persistent issue and got it sorted, to their credit. I didn't even buy the car from them, as it's a lease through my employer but still they were happy to help.