Thanks for the clarifications all, I was incorrect in thinking that you couldn't use an IMC on a non-EASA licence on an EASA aircraft. Apologies, and Item 5 on the interim statement clarifies that nicely.
As noted by others above, I'm going to attempt to err on the side of caution and get the IMC added to my existing JAA licence prior to the 30th June. (Although if not, I now understand I can get it added to a CAA licence post that date and use it until 2014).