Thanks both for your responses, i've tried both suggestions and it hasn't made a difference (the hosts were already set to 1024 ports). i've unprepared / re-prepared every host in the cluster in case
interestingly i do get an error when i try and start monitoring the port state on the portgroup - and only on the portgroup which contains the problem VM
Cannot complete a vSphere Distributed Switch operation for one or more host members
vDS operation failed on host xxx, got (vim.fault.VimFault) exception
I have googled the error but only found suggestions around Nexus 1000v which is not used in this environment.
Thanks
Marc