Two large off-white roses bushes, I planted from pots bought from garden centre 3 weeks ago. They have black spot on some leaves, but the flowers have gone droopy and misshapen and now dropped most petals. Is this fungus as well? They had spots on them too, pink or brown, not black. My wife said it was black spot when I got these.
Rose Black Spot
It sounds like you have both a fungus and insect problem. I would apply some Bayer Tree and Shrub granules around each of the roses as directed on the label. Then spray the rosebushes with a good fungicide that is listed to control black spot. If the black spot is in heavy attack mode, you may want to get a fungicide called Mancozeb at http://www.rosemania.com and spray the roses with it at least three times 5 to 7 days apart. It will leave a yellowish residue on the foliage, but that is how it works. Also, the black spots will not go away once they appear on the foliage, however the new foliage should be clean of it if the spraying has been successful.