David Hearn wrote:
I'm a little confused then - if the wall is a cavity wall - how could water
cross the cavity? Surely, a porous render is not much worse than no render
(apart from it possibly drying out slower) because its still got to go
through/into the bricks/mortar?

I expect that if the render soaks up water and stays wet, then that will
seep into the brick work underneath. If the brick was bare, then it
would stand a better chance of the water running off. Assuming that the
brick was okay underneath the render - No way of knowing until it's
stripped off.

I may have a complete mis-understanding - but I thought that a cavity wall
(unless bridged) shouldn't allow damp to pass through it?

I dunno. I'm surprised that that much damp can make it through the wall
as well. But I guess unless the cavity is a vacuum, water can still
bridge the gap. The air between the two layers of brick will become
saturated from the wet outer brick and transfer it to the drier inner
brick...? I guess the problem would be much worse without the cavity.

-Duncan