OK, so I managed to fix this in about 30 mins, as I can't stand anything broken - the design is a bit odd, as the two vents are clipped directly into the main dashboard moulding, rather than clip into a frame which then fits in to the dashboard - must be a design characteristic that is part of keeping the car so small.
Removal of the central air vents goes something like this:
Based on this diagram: http://mmc-manuals.ru/manuals/i-miev/on ... dex_M1.htm
under: 52a interior> INSTRUMENT PANEL ASSEMBLLY - and the two sub directories.
1. Remove the top plastic cover that contains the radio (that will be part 5 for LHD, and part 3 for RHD), lift up from the rear and it just pops out, then disconnect the stereo and remove the entire piece.
2. Remove the screw (f) on the 'under cover) - but only the one closest to the centre of the car (this allows the 'Instrument panel centre lower panel assembly' (9) as defined on the LHD, or the 'Centre lower panel' (7) as defined on the RHD to be unclipped at the top, and then lowered out of the way.
3, Part (9) for LHD / (7) for RHD is only retained by clips, which need to be gently pried off from the top first. Using a plastic pry tool, insert in the spaces in the top edge of that part gently until the retaining clips come free. Note that the retaining clips for this piece are tighter than the radio cowl
4. Now that the top of that piece is free, gently release the clips at the bottom of the piece by hand. As this piece is also retained on the drivers side by the 'Under cover' (part (7) LHD / part (5) RHD), you have to gently work at the 'Under cover' to free the lot. At this point, the entire sub-assembly can be lowered down from the top. The only reason you need to do this is because the hazard warning lamp switch needs to be removed.
5. Remove the hazard warning lamp switch - there is a clasp at the top and bottom that allows the unit to be removed from the front. Disconnect the switch once removed.
6. You can now pry the vents out, with a plastic pry tool. It is fixed to part of the single molded dashboard piece, so you wouldn't want to damage anything - though the plastic is strong, and it allows for the requisite movement. With care, pry it out with a plastic pry tool from the lug towards the centre of the car - its in there quite tightly actually.
7. Fix/ replacement is the same process in reverse.