Help with bodged window installation please

[Rocket Ron]

So we have bought a new house which is 100 years old and we have a plasterer booked for the 2 March to skim and plaster the old walls. I’ve been prepping by removing old plaster and discovered what I consider to be a bodged upvc window installl in the top floor.

 

they have used MDF liner's to fix the UPVC to and the gap between the liner and brick is 2.5 odd cm filled with expanding foam.

 

the external sill sits on another old wooden sill too.

 

what do I need to do to correct this ? Should I pull the window and use 18mm ply liners ? 

[Rocket Ron]

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