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Might just be the picture, but it looks like a brick-sized hole. Any reason you wouldn't just cement one in and plaster/fill over it for the internal side. Would be easy enough to remove when doing the outside if needed? That's what we were advised and have done for the internal side of a much-larger vent in the front upper room. We'll deal with it properly when we pull the cladding off the front of the house.
Other vents on the ground floor are slowly being unblocked. They have been filled with either cotton wool, expanding foam or rubble. The cotton wool is gross, the foam is crumbling away and rubble is... well... rubble...
Slowly making progress on our kitchen and the next job is to fill this vent before decorating. No cavity, looks like brick/blocks inside and bath stone on the outside. 90’s extension to a listed building. Anything wrong with this plan?...
For the inside wall... Fill the inside with a layer of cement, leave a centimetre from the wall surface. Once dry, cover with filler and sand back. Was going to leave empty space between front and back wall.
Outside... leave until spring when we might get experts in to clean the front of the house. Get them to deal with it so the colour matches the rest of the building.