Ah, I may have answered part of my own question.
The WSDL should contain its own location on the target server.
<wsdl:service name="xxxWebservices">
<wsdl:documentation>Collection of services provided by the xxx Web Service</wsdl:documentation>
<wsdl:port binding="tns:WebservicesSOAP" name="WebservicesSOAP">
<soap:address location="http://xxx,co.uk/services/Webservices"/>
</wsdl:port>
</wsdl:service>
The value held in LOCATION is used to populate the ENDPOINT value in the SoapUI test case.
This value tells the test where to run.
So in theory... if the WSDL moves to a new server, it should be updated with its new location.
Re-importing the WSDL should then update the tests to point to the new location.