I can second the comments from Tom having played a house concert in the last few months ; it was a pleasant experience, and there were certainly more than 6 people in the room at the time! (I think the venue normally gets between 20-40 turning up on average, depending on the nature and reputation of the guest booked). During the interval there was also provision of jacket potatoes/cottage cheese/ biscuits etc etc. No set charge for audience, just a donation into the communal pot. And advertising by means of a circulated email to those normally turning up. Certainly from the conversation I had with the organiser things are kept low key for all the reasons mentioned by Phil B. I have also been having an internet chat recently with someone who is intending to start up a house concert venue at some point in the not too distant future. So, as already stated, the venues exist, but have a very low (if not even flat) profile for many reasons.