Anyone got any tips here? I understand you cannot use wood fibre, which is a shame, as thats what I am using everywhere else

The issue clearly is water, both as high levels (condensing high) of vapour and as liquid water jets in places.

So you can use wood fibre, or any other hygroscopic, organic insulation, but you need to be careful. You need to think about how it will dry and how to protect it against liquid water in particular.

I'm mad. I have straw bales covered in lime plaster inside two of my showers. It seems to be OK. But one wall is quite far from the shower head and only gets a few drops splashed on it. I've covered that area with Stormdry clear coating. The wall in the other shower is a bit closer to the water, so I've put a waterproof roller blind on the ceiling so it can be rolled down to act as a rainscreen for the wall when we use that shower. Then rolled up afterwards to allow the wall to dry fully. MVHR helps a lot of course.

In general though, you might want to be a bit more conservative. Marmox do a whole load of XPS panels intended for insulating bathrooms, among other things. Bushboard do an XPS decorative panel for the surface, if that sort of thing appeals - we like them.

When you say insulate, what U value are you aiming at for the wall?

Is this retrofit and are you using a vapour-tight layer on the inside (as many but not all recommend for retrofit IWI)? If so then you can put whatever insulation you like outside of that.

For what its worth, we used aquapanel (printed plastic coated ply) as the decorative and waterproof layer, then PIR insulation, then a ventilated gap over existing stone wall. Floor is glazed tiles over Marmox insulation board over existing cement floor.

I just used PB pre stuck to XPS, tiled the bits that get water droplets on them (toilet, shower, sink backsplash) and I have MVHR. I stuck them to the wall with squirt foam and very few fittings. This IWI on a solid ultra porous stone wall, capillary open cement rendered externally with good masonry paint.

Delprado, just wondering what you did in the end?

Bathroom currently being ripped out in my own house at the moment, some of the old lath and plaster will have to come off where it is pulling off in chunks with the tiles and I am thinking about just pulling it all off from the external walls and insulating from the stone wall in. I'm a bit torn between using something impermeable to stop moisture getting through, and with keeping it vapour permeable. That bathroom was not well ventilated before we got here and most of the lime based plaster is still in very good condition despite being in a damp environment for ? however many years since a bathroom was added there which is at least 30 years and probably a lot longer.

Bog-standard nothing-special tile adhesive.