I have just gotten off the phone with support, and they have confirmed to me that the engineering team are currently aware of an issue with the latest version of the pvscsi driver.
Our entire estate of VMs uses the Paravirtual controller, and have the latest version of the driver installed.
They suggested the current workaround was to downgrade the version of the driver (one of the first things I tried as mentioned in one of my earlier replies), but then said that this doesn't always seem to resolve the performance problems.
My support ticket has been tagged against this issue, and I will be getting an update once the dev team have a fix, or release a new build not affected by this problem.
Hopefully most of you all also use the pvscsi controller, and this will be a little comforting to you to know they have now acknowledged the issue and are working on it.
James