Thank you very much for your help Zulu 17. Let's hope that an update is issued soon which corrects this problem.
All I can say is the situation is as before. The tennis replays on Amazon Prime do not work so I presume BTTV has not yet updated the application. Thanks for your interest and hopefully this will be sorted soon.
I was having the same issue but since i updated to the 4K box, the issue has gone away. From various forums, it doesn't seem to be isolated to Youview boxes as i've seen similar reports on some Smart TV's and other hardware. It appears to be a problem either within the App or with Amazons servers. Easiest way to resolve it until Amazon decide to do anything, is to turn the box off and on again and it will be ok for a few weeks.. We've got a FireStick and that is also notoriously rubbish on memory management with some apps, ie. Netflix...
I've not checked the tennis but the low bandwith issue isn't just affecting tennis to the best of my knowledge, but since changing my box and upgrading my package i've had no issues with Amazon Prime via Youview