Never heard of window installers breaking damproof course - more like it was never put in there in the first place! It would be in their interest that the windows did not leak - (did the builder fit the windows himself as well i wonder). Usually mastic is also put all around the edge of the window to help seal and then the plaster reveal covers this.
This property needs to be checked out big time, It might be worth looking for a thermostatic report to be done (to check out how the roof, insulation, windows, doors etc are preforming) think it is going to be regulation soon to have a rating on a house.
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