Internal insulation on External Wall

I have a traditional Scottish lath and plaster external wall house.
Construction is 9 inch brick with external render.
Then a ventilated cavity,around 25mm, behind the lath and plaster.
The wall has a 50mm kick in it between dining and kitchen areas meaning it would be easy to put insulation and plasterboard to a depth of 50mm onto that wall.
I was looking at Celotex PL4000 which has integrated 12.5mm plasterboard and 25mm PIR insulation giving an overall depth of 37.5mm-then plaster on top upto the 50mm level.
Do I need the plaster on top or would Ames taped joints be ok for analglypta paper?
If Ames tape I could use a deeper Celotex board with more insulation.
Not sure how to fix the Celotex-I won't be able to find the uprights in the lath so could I glue it on.Existing wall has wallpaper on the plaster-it seems sound or do I need to remove it before glueing Celotex.
Or do I use really long fixings screws or something to get through the Celotex and lath and plaster into the brick in the external wall but probably need to be about 6 inches.
Anyone done anything similar and how did it go.

Another

Have you watched Charliediy on YouTube.

He has a number of videos showing options but his most recent posted is probably the best way to insulate walls.

His first video he dot and dab insulation on the walls.

Maybe look at the different ways he uses.

Wayners said:

Have you watched Charliediy on YouTube.

He has a number of videos showing options but his most recent posted is probably the best way to insulate walls.

His first video he dot and dab insulation on the walls.

Maybe look at the different ways he uses.

Click to expand...

I notice how he warns folk about the unpredictability of foam (expansion) and the faff of trying to get stuff upright/level etc. It's not as easy as you think.
I'm a big fan of (low-ex) foam though when it comes to insulation and air tightness detailing.

Bobby12340 said:

I was looking at Celotex PL4000 which has integrated 12.5mm plasterboard and 25mm PIR insulation

Click to expand...

25mm of PIR won’t do much (though it’s better than nothing).

Bobby12340 said:

Do I need the plaster on top or would Ames taped joints be ok for analglypta paper?

Click to expand...

Either plaster skim, or tape-and-fill.

You do need to think about moisture. Is the wall damp?

Historic Scotland have a whole load of case studies about insulating old properties: