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to be honest it's proabbly a gap in your air barrier but as above the accepted standard in the uk is not a high level of air tightness. people just call it ventilation or something. you're not supposed to have air blowing behind your plasterboard and your insulation is supposed to be continuous. All this usually depends on the opinions of your builder. And he's probably seen victorian houses with no insulation and open fireplaces aplenty. If it were me I'd seal it up, might save a bit of gas and stop the draughts.