Bathroom render

Hi guys
We have a 1945 semi and have extended the bathroom 18 inches. Stripped everything out, old plaster skim damaged in places, back to brick.

Original intention to rip all old plaster off and Marmox board, however the original won't even come off with sds chisel (unlike the rest of the house which falls off easily).
Even behind the toilet, where lumps came off with the tiles, what's left is hard to remove so we have patches back to brick but not a full area to board.

Therefore, it may be best to render exposed brick and stick within 20kg tile limit.
sand and cement seems best, can I use this to patch what's there?

I usually use hardwall but s&c is better for bathrooms but is it ok with what's there and the 1940s mortar/brick?

It doesn't look quite as bad now as I've removed the old tiles, the toilet and green plaster you can see in the photos. (which is where the bath goes so this area isn't too important)

I wondered if a rough render of the holes, then using Marmox 6mm over it, Thanks muchly for any help from those that know!

 

Thanks CGN, Quick question, the existing bricks are 1940s so likely to be lime mortar? I read on a 2010 post by tic tic saying wet the walls, 1st coat 4:1:1 plastering sand : cement : plasticiser. 2nd coat 5 : 1 : 1 plastering sand : cement : lime.
Some places I only need the s@c to be about 10mm to match up with the existing render, should I just do sand : cement :lime or just s@c be ok?

Also, I'm guessing the s@c reacts ok with the old gypsum render.
If the tiles we choose are under 16kg+adhesive, I'll prime and tile straight onto this.

 

@Pollowick @KIAB thanks for your answers guys. If the tiles we choose are over 16kg+adh we'll do this. You're perfectly right, I got hold of the marmox pdf and if fitted on a full bed of cement based adhesive 6mm marmox can be used to cover the existing render (great suggestion on the Fischer steel washers, I'll check them out).

If I go the marmox route I'll still need to ensure the wall is as level as poss first so I'll do CGN's s@c render etc first. Really depends on the tiles now.

Maxmox does also avoid having to tank the shower area over the bath. If fixing marmox with cement based adhesive do the gypsum areas still need priming to stop a reaction with the cement adhesive?

 

Ah yeah where the white tiles are lumps cane off and it fell off up the side of the window. Also to the left of the pic showing where the bath was, it's bare brick for 18 inches. (Where we extended into the bedroom and it all fell off)
Cheers for your advice