Preserving whitespace in Run element

When formatting a paragraph as so -

<RichTextBlock>
<Paragraph xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'>

<Run></Run> 

<InlineUIContainer><TextBlock >Words here</TextBlock></InlineUIContainer> 

<Run xml:space="preserve"> </Run> 

<InlineUIContainer><TextBlock>More words here</TextBlock></InlineUIContainer> 

<Run xml:space="preserve"> Lastly some words here</Run>

</Paragraph>
</RichTextBlock>

The <Run> element containing a single space is never displayed and ignore the space=preserve property. However the last run with a leading space is displayed correctly.

How can I get a run containing a single space to display ?