How to insulate internal end of external wall

[Spinny]

See the sketch. I have the end of an external wall running into the internal area alongside a window.

I am wondering what level of insulation to use across the end of the spur of external wall (where the external wall was cut back to create an opening).

I can use some insulated plasterboard on the inside of the window reveal, but the end of the spur will be plastered over (wet plaster) with the adjacent brick wall. I guess I need a little bit of insulation between it and the plastering to avoid a possible cold spot in a utility room.

 

Suggestions ?

Thin piece of PIR ?

Something I could spray it with ?

Aerogel matting ?

 

[Spinny]

Should I use this stuff ?

 

https://www.insulationuk.co.uk/products/tekwarm-reveal-board-high-performance-laminate?variant=43387649261749&utm_medium=cpc&utm_source=google&utm_campaign=*Pmax|Ins.Plasterboard30+&gad_source=1&gbraid=0AAAAAoNNCLQ_tq0b6Prkw491vqllba1g2&gclid=EAIaIQobChMI7_mygKjujAMVrpFQBh2LXgvDEAQYASABEgJc7_D_BwE