152 Jock,
I tried the same thing you're considering about 7 or 8 years ago when the UK CAA licence was still in effect, pre-JAR, so not relevant now but I imagine the same sort pain is required to jump through their hoops.
I had 350TT, CPL, Multi IFR and because I had less than 700hrs I would only qualify as a Basic CPL (not a full CPL) which allowed you to do aerial photography and instructing I think?? I could do a short up-grade course of 109hrs to get a full CPL but I'd still have to do 14 exams, a CPL flight test & IFR flight test to complete conversion. I think the going rate for a Cherokee 140 was about GBP180/hr which at the time equated to nearly NZD550/hr for a single engine, anyway it came to nearly NZD60k to do everything they wanted so flagged until I had more hours. Still interested in doing it at some stage, especially if they relax the conversion requirements. Interestingly though, they did credit my Night rating and morse code (which I knew some of but not enough to be credited towards a licence!)
Anyway, I can't see it being a whole lot different now under JAR but am happy to be corrected.
You could try and enquire about CTC in Hamilton who have JAR instructors and examiners, who could do both for you prior to departure.
S2K