Thanks, Davyjones, I think I'm starting to accept it ( although it's not what I really wanted to hear!). Do you know anything about charcoal filters, are they any good, or any other alternatives for a room that has no outside walls? I don't think we can run a pipe through the floorboards of the room above because the roof of the lean-to extension comes up to about 20cm under the window sill of that room so the pipe would come out in the living part of the room!
I'm just not imaginative, I'm sure there's a solution but I can't think of one...