We were really surprised and I decided to wipe it off with water, but it only made it a lot worse.
In a panic I telephoned the company who sold us the £400 fire retardent paint, who said that we should have painted the varnish on before we let it get wet, and there was now nothing that we could do.
However, when I went back outside a while later, it all seemed to have dried and was looking much better. We then spent time experimenting with wiping it gently with water, sanding the damaged bit which definitely didn't work, and finally varnishing an area to see what happened.
Unbelievably, once we had varnished it, then most of the damage disappeared and it still looked good when we left tonight.
Hopefully it will still look good tomorrow and I will give it another coat.
From now on Darren will put up more cladding and then I will varnish it straight away, and we do have a heatwave coming up, so hopefully we can get to the top before it rains again.
Also, our next door neighbour spent a lot of time today levelling the fence, and Darren is going to put back the panels.