Hi Paul
My Pulsar observatory also suffers some condensation not so much as now in the autumn but more so as the temperature drops and dew appears on the grass and the devels hand shows on the last of the blackberrys. I bought pulsars floor kit consisting of a poluthene sheet and some cushoned interlocking rubber like flooring to go on top. Your first problem is undoutedly the concrete absorbing moisture and then releasing it during the night and condensing onto the roof. I have just looked inside my dome and currently there is no sign of any dew on the dome roof. Winter drawing closer will increase the risk and I now have an ecoair dehumidifier. The first step I suggest is to seal the concrete surface, you can get this from B&Q etc. and then cover the surface with a plastic sheeting to see how this goes. Some people use a fan heater to raise the dew point as an alternative to a dehumidifier.
Best of luck
Nick