Having spent the last few weeks conversing with the CAA and my potential future employer, I thought I'd share the knowledge for those interested who helped me get to the answer.
In my quest to find out whether I would need to have a current ME IR prior to starting a TR course (after EASA comes into effect and therefore discounting any QSP accreditation/"current and valid" rulings due to mil IR), the CAA stated that because the TR wouldn't be my first MPA TR (my first one was on mil MPA which opened my license) then I could renew my IR on the TR course. The only timeframe I had to meet was to renew the IR within 7 years of the expiry of my last civilian IR (otherwise face doing the 7 IR theory exams).
This news was gratefully received, however, my potential future employer stated that regardless of the CAA ruling, they wanted a current and valid civilian IR on arrival. Lesson learnt, ask everyone involved!