First of all much thanks for all your input.
From what you've all said my big concern is that despite being able to convert to a full ATPL it seems I would be forever trapped in the right-hand seat... not a thrilling thought... and unable to transfer across airlines. Do you think the aviation authorities under which I would hope to fly would answer this question directly? i.e. CASA? 27/09's reply was slightly too technical for me.
Thanks smiling_monkey for the contact- I'll be sure to mine them for info.
A new question I have relates to the traditional route of gaining the ATPL. Presume I knock out all the licences, ratings and endorsements: is there always a 5-10 year hold-up as you acquire hours enough to be considered by a reputable airline? If so, this seems to be a real positive for the MPL...?