No smokers, no cat any more (sadly missed), kitchen is three rooms away and that has an extractor fan. But we do (aha !) have a woodstove in the living room.
BTW when I bolted the Machine back together I found I had one USB cable left over without a socket to plug it into. Highly mysterious. Are aliens stealing USB sockets ?
Off to PC Box on Monday to buy an additional USB card to fit into the rear.
Thanks for everyone's suggestions.