It's got to have the same finish as the old cottage. You might be able to see it on the right side of the window.
Easy enough to do, just render the wall, don't use a straight edge and finish off with the sponge. Waterproofer added to the mix.
This type of cavity batt is designed not to be affected by moisture, as it's treated and the fibres all go the same way. It can be used where the outer skin is face brickwork.
There are other methods of achieving the required U values though. Another popular way is to part fill the cavity with rigid insulation boards.
There are also insulated plasterboards to fix to the interior walls instead of wet plaster. Another method is to use a 12 inch AAC block for a solid wall. The external is then protected with stucco, thin bricks etc.