I've recently upgraded my UK CPL to a JAR ATPL and have been told, by the holy CAA, that the required 'skills test' would be covered by my latest LPC/OPC-it involved nothing more than the standard requirements for the 6 monthly check. If the applicant held a UK CPL, then the skills test could be completed before or after the ATPL requirements had been met. However, if the applicant held a JAA licence, then the test must be completed AFTER the ATPL requirements had been met. The CAA found no problems with my application and I did not send in a OPC form.
I'm sure they make up their own rules on an as and when basis.
Hope this helps.