I contacted support and asked or the case to be escalated as I wasn't happy with the initial response and I'm still convinced the upgrade has caused the problem. There seem to be a number of posts on here where the problem started after an upgrade. I then had a response from a Veeam engineer who advised that a number of similar cases had been reported and Veeam were investigating. I wouldn't mind betting that there will be a fix for this at some point.
That being said, I decided to try one of the options advised by support:
Select Traffic Throttling from the Veeam Console > Un-tick "Use multiple upload streams per job"
So far, since this change, the issue has not re-occurred. Backups are taking about the same time as well. I'm monitoring this for a few more days before confirming all OK with support. So far so good!
Hopefully this bit of information may help someone else