Apologies, I thought I replied to this to say it looks like it was an issue with my PC after all. I’d not read that the old client was obsolete with 5.1 and couldn’t fathom out why it wouldn’t work until 2 reboots as it error’d with VPX issue the 1st reboot and also on the VC itself then it seems to have settled down. Thanks for the links, they may help others and also as ever the client is on the VCenter 5.1 ISO.
Im not too sure about the comment of both 5.0 and 5.1 clients residing on the same PC for when you connect to the Vcenter, it downloads and overwrites the old 5.0 client. Maybe you can install the new client before connecting to a 5.1 host and choose a separate install location but the 5.1 client is backwardly compatible so Im there isn’t much of a need for both. Thanks.