Here is my three ha'pence for what it is worth.
One of the reasons I come to the marlborough club is because you do allow smoking.If the room was very smoky people would have a point, but as it is a big room with plenty of ventilation and the amount of smokers are minimal I do not believe you have a problem.In fact it is a lot less smoky than a lot of pubs were they do have singarounds and there is no ban on them............yet.

Mrs.Ossonflags - a non smoker - hates smoky pubs/clubs with a passion, but she does not object to the amount of smoke in the Marlborough folk club.

I think if you are going to impose a no smoking ban - as they have at Cottingham - you should do it straight away, then people will know how they stand.