I have solved this problem:
To do so I had to solve it in a rather round about way:
1) I installed the Earliest Version of the standalone cvonverter that I could find on the VMWARE website (I think it was 4.0)
2) I converted the Vmware Virtual Computer to a NEW version 6.5 VMWARE virtual computer (which is the latest version that 4.0 will allow you to convert to)
3) I then uninstalled the 4.0 converter (you cant have two installed on the same computer)
4) I installed the latest 5.1 Standalone Converter
5) I imported my NEW version 6.5 VMWare virtual computer into my VMware infrastructure ESXi 5.1.0 system.
This worked on the 3 systems I tried it on. 2 Windows 2003 Standard 32bit servers and a Centos 5.5 Linux server.
Im happy that I have got all my systems migrated. I'm NOT happy that VMWARE is so cackhanded that you have to use an ANCIENT version to bootstrap into a new version. Surly a new version would still have the old code installed that could read old hardware images?
Anyway..
SOLVED