Not an expert by any means but it sounds like lack of consistent dry heating may be the issue. If I were tackling this I would get an electric dehumidifier, use for about a week in each room you feel is damp (you'll be amazed how much water is collected - also it's distilled/filtered water so good for things like washing windows with). Keep the heating on consistently in the house during this time, then paint the problems areas with damp proof paint. Damp proof paint is usually very thick and viscous to brush on but you can then paint over the top of it in your colour/paint of choice without problems. You can even use an anti mould paint for the top coat for an extra layer of protection though I've never found 'anti mould' to be as good as 'anti damp' paint. This may seem overboard but also introducing an airbrick and stove would make a huge difference. The airbrick would improve ventilation and the stove would dry out the house to prevent mould, mildew and damp. That said I'm a little surprised that an Edwardian house has bad air flow, it's usually the hermetically sealed newer homes that have a problem with ventilation. When we first bought our early 1930's house it had a damp/mould issue but since we've had a stove it's been brilliant at really drying the space inside out to prevent any reoccurrences.

Thank you for your comments. The article you posted was really interesting and pretty much sums up what is going on with the house. The downstairs floor are concrete and blackjacked as they have obviously taken up the flags, They are also uneven so will have to get taken up to level out. I'm hoping there isn't 10 inches of concrete but even so it's going to be difficult to take out. This has pushed water into the walls, so they then blackjacked the bottoms of those and this has pushed the water higher. If only any of this was present when I bought the house I won't have bought it. The builder they recommended that had done all the works was supposed to be a member of the Federation of Master Builders, but I remember looking him up when problems first started arising and he wasn't listed. We have started stripped all the plaster from the external wall and the room is starting to get drier already. The walls are no longer sweating. I'm trying to work out what is under the blackjack. It doesn't look like plaster and I'm wondering it they have put some sort of cement based render on the stonework. Either way it's all going to have to be done correctly now. Bye Bye Bank Account :-(

Hi! It's not essential to remove the black mould first, but you'd probably want to do that first. Tiles are really trick when it comes to mould and mildew, a lot of the time it's not about completely preventing it but staying on top of it. Having a plastic trim will definitely help with containing the issue, but maybe look at getting a plumber to seal it with some sealant. There's a section in this blog post about cleaning mould off of sealant that could be helpful if you wanted to take a read of it.

I cannot comment on anti mould paint as I have never used it, but I found this interesting article which may help. I always open a window after showering to get rid of the steam and I also do that in the kitchen whilst cooking as I don’t have an extractor hood.