Not enough imformation .you cant skim on walls with hair line cracks as they will come bcck and haunt you, you need to adress the cracks.Dont understand the plastic "seething" bit! Exposed brick doesnt tell us much...Is there a cavity ?Have you got any damp problems?
What you have to do here is knock off all the old plaster on front of brest and sides.Then cut a square hole in the thermalite blocks in the centre of the third coarse up say 6"across by 4"down to accomondate a 9×6 plastic hit and miss vent.But first you have to overlap the therms and concrete lintol all around with stainless steel mesh by 6 inches and then set your corner beads up and scratch coat the brest then float it off and devil it then skim it. Then the job is a "good un".lf you dont understand this then get a plasterer in to sort it out..
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