If you want to modify a property group definition, yon only can add more element but not remove or modify them once registered.
I have finally managed to get the property groups working with the option's I wanted.

1) If you have already created a property group you CANNOT modify it, so if you are having problems start from scratch.
2) When adding the PropertyGroups.xml finle I still got the error message 'Missing = in namespace decl. (CustomNodes.cnd, line 1)' however the property groups seem to work perfectly. the error message is not important (as far as I can see)
3) In the main directory of your OKM installation you should find PropertyGoups.cnd and PropertyGoups.xml, I modified these files and then added them through the administrator console.
4) Make sure your files are 100% correct, no Capital letters or or missing characters. If you get it wrong you'll probably have to start all over again with a new installation!
5) If you are having difficulty check you can use the demo PropertyGroups files to create your property groups, they can be found here: http://wiki.openkm.com/index.php/Proper ... definition
Below I have included the actual working PropertyGroups files I have used to get the options working as I needed.
Hope this helps anyone having the same trouble, and also a BIG thank you to all who gave advice.
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