Hi there - I'm a mould remediation professional happy to give some tips to consider...
The first thing to do is to focus on the cause of the mould, which you have started to do. Otherwise, removal may only be temporary and the mould can return if the conditions stay the same. Without knowing much about the property...
Given it is in a bathroom, it's pretty clear that the moisture will be coming from bathing and showering. This water vapour will likely be settling on surfaces as moisture (which mould needs to grow). Mould also needs - an organic surface (such as plaster) to grow on, the right temperature, lack of disturbance, lack of air movement, it prefers not to have direct sunlight too. Of all those things, poor ventilation is most commonly the main cause but the more of the other things you can help with the better the chance you have of getting on top of it.
You have identified the ventilation issue and trickle vents will very likely help. Think about seeing if the extractor fan (if there is one) is working well and big enough for the room as that is a big help. Also, is it set to run a few minutes after use if you can.
If you are able to in the meantime and it is safe to do so, especially as the weather is not cold, try to keep windows open for a while after a bath or shower - ideally (and again if practical and safe) when the window is open and you have finished the bath or shower, keep the bathroom door closed so that the steam does not go into the bedrooms and cause problems there.
As an aside, if your room is north facing, it is always going to be more likely to be at risk of mould problems.
This article about
mould on walls, might provide some other useful information for you.
Thanks and good luck.
