You can try giving them a lot of water both on the foliage and in the ground but I suspect you have killed your roses.

Once you get it off the leaves, spread about 1/4 kg of wettable sulfur for every 9 sq. m. of area under the roses, and cover with 1 cm of composted manure, then water very, very deeply. That will counteract the chlorates in the soil. In the UK, you may want to follow that up with an application of lime, unless you are on limestone soil. Good luck!