Hello Gareth,
if you're thinking about using BAPIs and extracting them from IDOC function modules you may also want to have a look at the Enterprise Services provided by SAP. You'll find a list of them unter http://esworkplace.sap.com. You can filter by business object and look for the service you need. Watch out for the EhP of your ECC and check whether that service is available on your system.
The advantage is that these services are delivered with complete SAP content, so you only have to do configuration in PI and expose the webservice to the consumer.
Regards,
Jörg