SQL 2008 R2 performance problems - mixed signals on disk performance
Not wanting to be one of those OPs who just disappears without providing closure here's an update..

Despite receiving multiple assurances from infra that the vhost was in no way overloaded and that contention for physical resources couldn't possibly be the problem they eventually moved multiple VMs off the host to another one and the symptoms disappeared once that was complete. So either the problems were caused by resource contention on the host bottlenecking the VM's performance or the symptoms that just coincidentally looked like resource contention just so happened to coincidentally disappear at the same time that load on the vhost was reduced.

So many thanks to everyone for the helpful comments/pointers!