Small 1500mm² Vents

Builders merchants, you can get 9 X 3 plastic vents, about the size of a brick, or you can use clay ones....I don't think a square one would ever look neat and the round ones are postively revolting

Err for wood floor so brass, and err 3" x 3" okay so I left the brass bit out

Why are you putting it in the floor? - this means you'll have to cut the carpet and also rely on other vents not blocking that are 'out of sight', namely the ones under the floor. This is not a risk I would be prepared to take.

No carpet, just plain boards. Already one in there that was larger for the original back boiler. Safety wise, I wouldn't have one at all and this is only for when I sell. I'll probably seal it up below the grill for my use. If you think that there isn't well over 1500mm² of ventilation in this room in my draughty house you'd be mistaken. And willfully adding more ventialtion when I'm doing my level best to reduce it would be daft. If and when I'm in any doubt about the available oxygen I'll consider more carefully. I'll probably have a CO detector given that solid fuel appliances don't have CO detecting shut off gizmos like their gas counterparts.