Jim W writes

A further question would be why do you have algae on your walls? Is the
DPC and insulation OK?

I've just cleaned the algae off my north facing sheltered wall of the
house, using just a pressure hose and it has new dpc and excellent
insulation below and in the walls.

The greening builds up because the outside surface of the wall is often
damp from dew and mist, sometimes all day or several days in a row and
because the wall is well insulated its not dried off from conducted heat
inside, nor from air movement as its sheltered. You can actually see the
vertical strips of the studs behind the plaster in between the patches,
presumably the solid wood conducts more heat.

At some stage it will be repainted again, and there is a fungicide in
the paint IIRC.
--
David