Thanks for getting back Woonsan. I have managed to achieve what I want looking at various stackoverflow answers and combining ideas.
For anybody else having difficulty here is what I ended up with:
<#assign isTrue=true>
<#assign radioButton=document.selection_booleanradiogroup>
<#if radioButton??>
<#if radioButton?is_boolean>
<#assign isTrue=radioButton>
<#assign falseLabelSelected='False'>
<#assign trueLabelSelected='True'>
${isTrue?then(falseLabelSelected, trueLabelSelected)}
</#if>
</#if>