How about 120mm of eps on the outside and then lime rendered onto metal lathing or mini grid?

I know that eps (expanded polystyrene) is not natural but it is a good use of an oil based product and it wont go rotten.

50x35 battens, multifoil insulation, 50x35 counterbattens, feather edged cedar boarding - luvvly, weighs nothing. Or rendermesh and roughcast lime render, ventilation up behind it. Gives full-heating-season heat retention equivalent to 200-250 mineral wool. Total thickness 95mm.

If, with top-notch wall /roof and window insulation, airtightness and controlled, preferably heat-recovery ventilation, your bathroom extension will be kept warm 24hrs, then such external insulation/cladding on internally plastered single-skin blockwork will be ideal - heavy(ish) interior. With Structural Engineer input, that could be 140 or even 100 blockwork - how easy is that?! If it's just to be warmed-up as needed, then you need a lightweight interior - the cladding, uninsulated, on the blockwork, then battens, multifoil, counterbattens, plasterboard internally. Or the cladding on say 120x50 studwork structure with 75 Cellotex between, and airtight internal multifoil, counterbattens and plasterboard. Until a few days ago - see http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=7#Item_5 - and maybe even now - for a newbuild extension, relying on multifoil alone for insulation would have been difficult to get past the Building Inspector - but no problem with thus improving the existing house, if his jurisdiction doesn't cover that. However he would accept the studded Cellotex-plus-multifoil solution. Any other questions, ask.