One of the pre requisites is that Dynamics GP 2010 web services are installed before business portal 5. Having followed the wizard I was convinced that the Dynamics GP 2010 web services were installed , however the business portal 5 installation wizard could not see them.
However this was not working.
The install documentation suggests the URL for Dynamics GP 2010 Web services should include the TCP/IP port (the default for Dynamics GP 2010 Web Services being 48620) as well as the web services being called DynamicsGPService.
This does work and you can continue with your Business portal 5 installation for dynamics GP 2010.