The metre high render is an impervious coating which could bridge moisture in the wall from below the DPC to the wall above, but it has probably existed like that for a long time, so it is unlikely to suddenly be causing a problem. It will however make the wall colder, so internal condensation is more likely.
Condensation and mould is a particular problem in the south this winter because of the weather.
The rule is allow ventilation and increase air movement, and avoiding intermittent or excessive heating during the months of October to March.
Condensation occurs when the air inside is warmer than the cold wall surfaces, and the temperature of the walls is below the dew point, so that moisture condenses on them.
If the temperature of the walls are the same as the air, then condensation cannot occur. Therefore keeping air temperatures reasonably low and steady, and moving air around, like with a fan, will bring the temperatures of surfaces into line with the air, and help evaporation of the dampness.
Ventilate when the weather is dry, not damp.
Do not heat intermittently and raise the temperature too quickly, or cold surfaces will not have chance to warm and will attract condensation.
Clean the mould off with a biocidal solution like antibacterial surface cleaner, mould remover, or my choice, thick bleach, do not scrub, lightly wipe or brush on the affected areas. If the mould leaves stains, thick bleach may be brushed on and left to dry. Keep the area ventilated until the smells diminish.
Bleach will also kill mould in bathroom tile grout. Lavatory cisterns and cold pipes are condensation sources. Try not to keep bathroom too warm. Ventilate/extract bathrooms during & after showering, and wipe down surfaces afterwards. Do not leave door to bathroom open.
Check behind bath panelling for hidden damp causing odours, find source of water ingress and seal with the correct type of silicone sealant.
If the areas remains dry, the mould will not return.