Sql Statement for a particular xpath

Question

I have a XPath statement that works well on my xml.

//PaymentProperty[Name='PaymentReference']/StoredStringValue

When I store this xml in mytable.xmlField, is there a way to get this value from SQL? I tried

select  x.xml.query('//PaymentProperty[@Name=PaymentReference]/StoredStringValue') from (select cast(xmlField as xml) as xml from mytable

But I guess XQuery likes different syntax? It is quite a dynamic xml so all I need to know is if there is Value of "PaymentReference" anywhere in the parentNode PaymentProperty against the tag then get the value against the tag StoredStringValue. Is there anyway to achieve this from SQL?

Thanks for all the help