Camposol

Air the place out every day...open all windows and doors for at least 2 hrs every day.

Are you using portable gas heaters ??

If so stop using them.

Too damp and not enough ventilation. If there's a window, has a passive vents installed in the frame and their open? Is there's an active ventilation while using the bathroom? Badly insulated walls especially around door and window frame and lack of DPC above the ground level. Mould growing should be tackled in localized areas preventing spores spreading in the air you're breathing in the rest of the house. 

If you're living in a MASA built house they are not designed for year round living. The walls are solid concrete and someone should have advised you of that when you bought it. The mould is a result of condensation which is made worse when cooking or using gas heaters. Even breathing within a small confined space will produce condensation.

You have several options depending on how much money you want to spend:

1,   Consider getting a dual role  air conditioning unit that can heat as well as cool, somewhere around e800

or

2.  Get some electric radiators, set them on a timer and use cheap rate electricity after midnight.  Much cheaper to buy.  If you get lucky you might find one in the charity shops.  If you ask around you should get some places to look in, Euronics on B sector spring to mind, Visanfer and Leroy Merlin are also possible

Try using Cillit Bang or similar mould killer to get rid of the mould and keep the walls and floor clear so that the air can circulate.  If you want to get creative - solar water heaters are now required on new houses?

Should have also added consider a gas boiler and radiators, with all the condensation being vented outside..  Gas is cheaper than electricity in Spain.  Solar panels are coming down in price, so there are lots of possibilities opening up.

Merry Christmas you too!