If you're referring to Black Spot, which is probably the most common fungal ailment affecting roses, you could use lime sulfur. It's generally marketed as Dormant Oil. It should be used first thing in the spring just as the earliest new growth is noted, primarily as a preventive measure. It can be used later when Black Spot (or Powdery Mildew) may appear, but it's rather hard on the roses and it smells terrible.
I use any product with 40% Neem Oil as a cure for fungal attacks as well as mites and aphids. It's made from the seeds of the Neem tree. The seeds are harvested in South-east Asia and is very effective as well as organic. Here in the U.S. my preferred brand is Green Lite. I'm sure there are many brands available in the U.K.
Good luck!