A, thanks for your reply.
I had already tried a "services.sh restart" and it didn't clear the performance counters.
As you maybe able to see the Highest Latency figure highlighted is 1.15x10^15ms which means the graph has scaled 1.29x10^9ms on the right hand side. This makes any useful measurement using this performance counter impossible.
I would like to clear this counter if that means that I lose all performance data for all counters on this host, that is OK, but what I am not able to do right now is reboot the host or the guest 
(Maintainance window is not till the end of the month, and would that clear the data anyway?).
Regards,
Jim