@john_smith - Thanks for the very useful and interesting answer. So when you get the ATPL, you only transfer across the TR? or the class rating also? (is there even an MEP class rating on an MPL license?), what about night rating for example? I was under the impression that there was still a certain training syllabus that has to be 'ticked off' in order to undergo the tests - or is that purely at the discretion of the ATO, even for the IR (single pilot) and CPL?
Say you already had an SEP (as in my case for example - i have a PPL), and you kept it current, when you apply for the ATPL could you add the SEP at that time or would have to re-do the skills test also?
Presumably there are very few if any people who have ever done this so far. I wonder if it would actually be quite difficult for an MPL to pick up light aircraft flying for the CPL/IR - its obviously fairly different to autopilot/auto-trim!