Thanks for the reply and your suggestions.
We already have the waterproofing insulation on the top of the various sections of roofing, so we know that there is no ingress of water due to the porous nature of the concrete. The reason that I am thinking of poly tiles, is that I know well of the insulation properties of them, so I believe that this solution will work. I remember many years ago, having them on the roof of a summerhouse and when, after several years, one fell off in winter, the following morning the frost could be seen across the whole roof, except where that tile was missing on the inside, showing just how much heat had escaped through that exact place, but nowhere else. As for them being unsightly, that is a matter of opinion and they are far more decorative these days than they used to be.
I have already investigated the mould resistant paint option, but have been told that it has to be renewed each year in order to continue working. Whether that is true or not, I don't know for certain, but the manufacturers only give a 1 year warranty, so it probably is. Obviously, that is quite a nuisance having to remove all the furniture from each room, paint it, let it dry and then go to the next. It is quite a big, single storey house, so I am looking for something more permanent.
As for ventilation, we have plenty of windows that are regularly open and the heating is electric, so it is dry heat and not producing the levels of moisture that you'd get with gas or oil burners, so it is a relatively small amount of condensation due to differing temperatures on cold surfaces, rather than produced by heating appliances.