Using Write XML action to create an element if not found?

[DRKev]

I'm trying to use the WriteXML action to create an element if it does not exist, without much luck.

The option under the XPath tab to "Create element/attribute if it doesn't exist" is checked.

I'm trying to write a simple log file to record information for specific versions:-

<VersionLog>
    <Version versionNumber="0.1.0.0>
        <Info someValue="blah"/>
    </Version>
</VersionLog>

If I set the XPath query to (and existing entry):
/VersionLog/Version[[@versionNumber='0.1.0.0']]
I then set the attribute to update to 'versionNumber' and try to assign the value '0.1.0.0'

I am told "File already up-to-date". This therefore indicates that the XPath is at least valid and that a matching entry already exists.

If I try to add a new entry for version 2.0.0.0, and change the XPath and new value accordingly, the step produces an error:
XPath expression not found.

This is not intuitive as it implies there is no XPath expression, which of course there is. There is no matching element, but I requested that a new item be created.

Have I missed something?
The documentation is rather sparse for XML with only a few very simple examples.
Does the WriteXML action support this simple request?