All the aquapanel will be on existing studwork with the plasterboard removed
OK, just checking if you were planning fixing panels direct to walls.
I was going to board floor to ceiling, but a neighbour thought it pointless and reckons people don't these days, but your right about loosing "land" on the bath.
Your neighbour is just being a tight asre
If you don’t board under the bath & water gets in, it’ll wreck the studding, it’ll also reduce sound insulation between any adjacent room.
B Regs now stipulate minimum sound attenuation/insulation requirements in stud walls for a newly formed/converted bathroom & w/c where it’s adjacent to a habitable room. It sounds like yours is a refurbishment of existing so not compulsory but, as you’ve got the studs open, it might be worth considering for the small additional cost. Cement based boards (Aquapanel) conform but standard wallboard spec isn’t high enough unless you double it up so you need an acoustic rated PB. You also need a min 25mm mineral wool insulation within the stud but I use cavity insulation batts.
The remaining walls are block, and are cement scratchcoat and skim. The skim is popping off very easily in places, so I was intending on removing the skim from the scratchcoat and tiling directly onto that, with a primer of somesort if required (that was my next question). This is the outside walls with the sink, WC and towel rad on - No shower or bath.
Plaster is not an ideal tile base due to weight restrictions & cement render is much better as long as it’s sound. It needs to be flat if your planning large format tiles; the larger the tiles, the flatter the base needs to be. I assume the outside walls are cavity?
The type of adhesive, grout & any primer you may need depends on the tile base & type, size/thickness/weight of your proposed tiles? With priming, you should always follow your chosen tile adhesive manufacturer’s instructions but if in doubt use an SBR/acrylic tile primer, never use standard PVA. Only use quality trade tiling materials, cheap DIY products are mostly inferior & can give problems.